System and method for liquefaction of natural gas

ABSTRACT

By using the power generated by an expander by an expansion of material gas, the outlet pressure of a compressor is increased, and a requirement on the cooling capacity of a cooler is reduced. The liquefaction system ( 1 ) for natural gas comprises a first expander ( 3 ) for generating power by using natural gas under pressure as material gas; a first cooling unit ( 11, 12 ) for cooling the material gas depressurized by expansion in the first expander; a distillation unit ( 15 ) for reducing or eliminating a heavy component in the material gas by distilling the material gas cooled by the first cooling unit; a first compressor ( 4 ) for compressing the material gas from which the heavy component was reduced or eliminated by the distillation unit by using power generated in the first expander; and a liquefaction unit ( 21 ) for liquefying the material gas compressed by the first compressor by exchanging heat with a refrigerant.

TECHNICAL FIELD

The present invention relates to a system and a method for theliquefaction of natural gas for producing liquefied natural gas bycooling natural gas.

BACKGROUND ART

Natural gas obtained from gas fields is liquefied in a liquefactionplant so that the gas may be stored and transported in liquid form.Cooled to about −162 degrees Celsius, the liquid natural gas has asignificantly reduced volume as compared to gaseous natural gas, and isnot required to be stored under a high pressure. The natural gasliquefaction process at the same time removes impurities such as water,acid gases and mercury contained in the mined natural gas, and afterheavier components having relatively high freezing points (C5+hydrocarbons such as benzene, pentane and other heavier hydrocarbons areremoved, the natural gas is liquefied.

Various technologies have been developed for liquefying natural gas,including those based on expansion processes using expansion valves andturbines and heat exchange processes using low boiling pointrefrigerants (such as light hydrocarbons such as methane, ethane andpropane). For instance, a certain known natural gas liquefaction system(See Patent Document 1) comprises a cooling unit for cooling natural gasfrom which impurities are removed, an expansion unit for isentropicallyexpanding the cooled natural gas, a distillation unit for distilling thenatural gas depressurized by the expansion unit at a pressure lower thanthe critical pressures of methane and heavier contents, a compressor forcompressing the distilled gas from the distillation unit by using theshaft output from the expander, and a liquefaction unit for liquefyingthe distilled gas compressed by the compressor by exchanging heat with amixed refrigerant.

PRIOR ART DOCUMENT (S) Patent Document(s)

Patent Document 1: U.S. Pat. No. 4,065,278

SUMMARY OF THE INVENTION Task to be Accomplished by the Invention

In the conventional liquefaction systems for natural gas such as the onedisclosed in Patent Document 1, the outlet pressure of the compressor(or the pressure of the feedstock gas that is to be introduced into theliquefaction unit) is desired to be as high as possible in order toreduce the load on the liquefaction unit (in particular, the main heatexchanger thereof) and maximize the efficiency of the liquefactionprocess.

In order to increase the outlet pressure of the compressor, acorrespondingly large power is required. However, in the conventionalarrangement where the feedstock gas cooled by a cooling unit is expandedby an expander, the power produced from the expander is limited, and isinadequate for increasing the outlet pressure of the compressor to thedesired level.

In the conventional arrangement, because the feedstock gas is requiredto be cooled before being expanded in the expander, a relatively largecapacity is required for the cooling unit, and this increases theinitial costs and the running costs of the cooling unit.

In the conventional arrangement, because cooling of the feedstock gaswill cause condensates to be produced, it is necessary to provide agas-liquid separator to separate (remove) condensates from the feedstockgas before introducing the feedstock gas from the cooling unit to theexpander. Furthermore, because the temperature of the feedstock gas atthe outlet end of the compressor is high, a significant temperaturedifference arises between the intermediate inlet point of the liquefyingunit and the refrigerant so that a correspondingly high capacity isrequired for the cooling unit.

In view of such problems of the prior art, a primary object of thepresent invention is to provide a system and a method for theliquefaction of natural gas which can increase the pressure at theoutlet end of the compressor by using the power generated in theexpander by the expansion of the feedstock gas, and minimize the coolingcapacity that is required for the cooling unit.

Means to Accomplish the Task

A first aspect of the present invention provides a system (1) for theliquefaction of natural gas that cools the natural gas to produceliquefied natural gas, comprising: a first expander (3) for generatingpower by expanding natural gas under pressure as material gas; a firstcooling unit (11, 12) for cooling the material gas depressurized byexpansion in the first expander; a distillation unit (15) for reducingor eliminating a heavy component in the material gas by distilling thematerial gas cooled by the first cooling unit; a first compressor (4)for compressing the material gas from which the heavy component wasreduced or eliminated by the distillation unit by using the powergenerated in the first expander; and a liquefaction unit (21) forliquefying the material gas compressed by the first compressor byexchanging heat with a refrigerant.

According to the first aspect of the present invention, the system forthe liquefaction of natural gas allows the outlet pressure of the firstcompressor to be increased and the cooling capacity required for thefirst cooling unit to be reduced by making use of the power generated bythe first expander owing to the expansion of the material gas beforebeing cooled by the first cooling unit.

A second aspect of the present invention further comprises a secondcooling unit (85) placed between the first compressor and theliquefaction unit to cool the material gas compressed by the firstcompressor.

According to the second aspect of the present invention, by increasingthe pressure of the material gas that is introduced into theliquefaction unit, even when the temperature level of the material gasshould exceed an appropriate range, owing to the cooling in the secondcooling unit, the temperature level of the material gas can be adjustedto a level close to the temperature level at the introduction point inthe liquefaction unit so that the load on the liquefaction unit can bereduced and the efficiency of the liquefaction process can be increased.

A third aspect of the present invention provides a system for theliquefaction of natural gas, wherein the liquefaction unit comprises aspool-wound heat exchanger, and the material gas expelled from the firstcompressor is introduced into a warm region (Z1) of the spool-wound heatexchanger located on a hot side of the spool-wound heat exchanger.

According to the third aspect of the present invention, if thetemperature of the material gas should increase owing to the increase inthe outlet pressure of the first compressor, by introducing the materialgas from the side of the warm region (Z1) of the spool-wound heatexchanger to bring the temperature level of the material gas closer tothe temperature in the liquefaction unit, the load on the liquefactionunit can be reduced, and the efficiency of the liquefaction process canbe increased.

A fourth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second compressor (75)placed between the first compressor and the liquefaction unit forcompressing the material gas expelled from the first compressor.

According to the fourth aspect of the present invention, the pressure ofthe material gas that is introduced into the liquefaction unit can beincreased even further so that the efficiency of the liquefactionprocess performed in the liquefaction unit can be increased.

A fifth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a first electric motor(81) powered by an external electric power and controlled in dependenceon a pressure value of the material gas introduced into the liquefactionunit, and the second compressor is driven by the first electric motor.

According to the fifth aspect of the present invention, the pressure ofthe material gas that is introduced into the liquefaction unit can beincreased in a stable manner so that the temperature of the material gascan be maintained within an appropriate range and the liquefactionprocess can be performed in the liquefaction unit in a both efficientand stable manner.

A sixth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second cooling unit(85) placed between the second compressor and the liquefaction unit tocool the material gas.

According to the sixth aspect of the present invention, by increasingthe pressure of the material gas that is introduced into theliquefaction unit, even when the temperature level of the material gasshould exceed an appropriate range, owing to the cooling in the secondcooling unit, the temperature level of the material gas can be adjustedto a level close to the temperature level at the introduction point inthe liquefaction unit so that the load on the liquefaction unit can bereduced, and the efficiency of the liquefaction process can beincreased.

A seventh aspect of the present invention provides a system for theliquefaction of natural gas, further comprising an electric generatorunit (87) for converting the power generated by the first expander intoelectric power and a second electric motor (84) for driving the firstcompressor, the second electric motor being powered by electric powergenerated by the electric generator unit.

According to the seventh aspect of the present invention, the firstexpander and the first compressor are electrically connected to eachother so that the outlet pressure of the first compressor can beincreased by making use of the power generated by the first expander. Atthe same time, the freedom in the mode of operation of the system can beincreased as compared to the case where the first expander and the firstcompressor are mechanically connected to each other.

An eighth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second electric motor(84) mechanically coupling the first expander and the first compressorto each other and powered by external electric power, wherein the firstcompressor is configured to compress the material gas by using powergenerated by the first expander and the power generated by the secondelectric motor.

According to the eighth aspect of the present invention, the powerprovided by the second electric motor can be used for augmenting thepower provided by the first expander in driving the first compressor sothat the outlet pressure of the first compressor can be increased in aboth efficient and stable manner.

A ninth aspect of the present invention provides a system for theliquefaction of natural gas, wherein the material gas from which theheavy component is reduced or eliminated by the distillation unit isdirectly introduced into the first compressor, and the system furthercomprises a first gas-liquid separation vessel (23) for receiving thematerial gas compressed by the first compressor via the liquefactionunit; and wherein a gas phase component of the material gas separated inthe first gas-liquid separation vessel is introduced into theliquefaction unit once again, and a liquid phase component of thematerial gas is recirculated to the distillation unit.

According to the ninth aspect of the present invention, the need for apump for recirculating the material gas from the first gas-liquidseparation vessel to the distillation unit can be eliminated, and thiscontributes to the simplification of the system.

A tenth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second cooling unit(85) placed between the first compressor and the first gas-liquidseparation vessel to cool the material gas.

According to the tenth aspect of the present invention, even when thetemperature level of the material gas that is compressed by the firstcompressor should exceed an appropriate range, owing to the cooling inthe second cooling unit, the temperature level of the material gas canbe adjusted to a level close to the temperature level at theintroduction point in the liquefaction unit so that the load on theliquefaction unit can be reduced, and the efficiency of the liquefactionprocess can be increased.

An eleventh aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second expander (3 b)placed between the first expander (3 a) and the distillation unit togenerate power by expanding the material gas, and a third compressor (4b) placed between the distillation unit and the first compressor (4 a)to compress the material gas distilled by the distillation unit by usingthe power generated by the second expander.

According to the eleventh aspect of the present invention, byadvantageously expanding the material gas in the first and secondexpanders, the cooling capacity required for the first cooling unit canbe reduced, and by using the first and third compressors that make useof the power generated by the first and second expanders, the pressureof the material gas that is introduced into the liquefaction unit can beeffectively increased.

A twelfth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second expander (3 b)placed in parallel with the first expander (3 a) to generate power byexpanding the material gas, and a third compressor (4 b) placed betweenthe distillation unit and the first compressor (4 a) to compress thematerial gas distilled by the distillation unit by using the powergenerated by the second expander.

According to the twelfth aspect of the present invention, even when thevolume of the material gas introduced into the liquefaction systemshould increase, the liquefaction process in the liquefaction unit canbe performed in a stable manner.

A thirteenth aspect of the present invention provides a system for theliquefaction of natural gas, wherein the liquefaction unit comprises aplate-fin heat exchanger.

According to the thirteenth aspect of the present invention, even whenthe temperature level of the material gas that is compressed by thefirst compressor should rise with the rise in the pressure thereof, thepoint of introduction into the liquefaction unit (the temperature levelon the side of the liquefaction unit) can be changed in response to therise in the temperature of the material gas with ease.

A fourteenth aspect of the present invention provides a system for theliquefaction of natural gas, wherein the material gas compressed by thefirst compressor has a pressure higher than 5,171 kPaA.

A fifteenth aspect of the present invention provides a system for theliquefaction of natural gas, wherein the material gas compressed by thesecond expander has a pressure higher than 5,171 kPaA.

According to the fourteenth or fifteenth aspect of the presentinvention, by raising the pressure of the material gas that isintroduced into the liquefaction unit to an appropriate value, theefficiency of the liquefaction process in the liquefaction unit can beincreased.

A sixteenth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a heat exchanger (69)for exchanging heat between the material gas introduced into thedistillation unit and a top fraction from the distillation unit.

According to the sixteenth aspect of the present invention, even whenthe temperature of the material gas that is introduced into theliquefaction unit is lower than an appropriate range, the temperature ofthe material gas can be brought close the temperature at the inlet endof the liquefaction unit by heating the top fraction of the distillationunit by exchanging heat with the material gas that is introduced intothe distillation unit.

A seventeenth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a first gas-liquidseparation vessel (23) for receiving a top fraction from thedistillation unit, and a third cooling unit (86) placed between thedistillation unit and the first gas-liquid separation vessel to cool thetop fraction from the distillation unit.

According to the seventeenth aspect of the present invention, the needto cooling the material gas that is to be introduced into the firstgas-liquid separation vessel by using the liquefaction unit iseliminated so that the load on the liquefaction unit is reduced.

An eighteenth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a second heat exchanger(79) for exchanging heat between the material gas to be introduced intothe first compressor and the material gas compressed by the firstcompressor.

According to the eighteenth aspect of the present invention, even whenthe temperature of the material gas that is compressed by the firstcompressor and introduced into the liquefaction unit is higher than anappropriate range, the temperature of the material gas can be broughtclose the temperature at the inlet end of the liquefaction unit bycooling the material gas from the first compressor by exchanging heatwith the material gas that is introduced into the first compressor.

A nineteenth aspect of the present invention provides a system for theliquefaction of natural gas, comprising a fifth cooling unit (80) forcooling the material gas compressed by the first compressor at a pointupstream of the second heat exchanger by using a water, air or a propanerefrigerant.

According to the nineteenth aspect of the present invention, even whenthe temperature of the material gas that is compressed by the firstcompressor and introduced into the liquefaction unit is higher than anappropriate range, the temperature of the material gas can be broughtclose the temperature at the inlet end of the liquefaction unit bycooling the material gas from the first compressor by using the fifthcooling unit. In particular, by cooling the material gas with propanehaving a relatively high cooling capacity, the freedom in the operationof the compression process of the material gas with the first compressorcan be enhanced.

A twentieth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a third heat exchanger(100) for exchanging heat between the material gas compressed by thefirst compressor and the top fraction from the distillation unit.

According to the twentieth aspect of the present invention, even whenthe temperature of the material gas that is compressed by the firstcompressor and introduced into the liquefaction unit is higher than anappropriate range, the temperature of the material gas can be broughtclose the temperature at the inlet end of the liquefaction unit bycooling the material gas from the first compressor by exchanging heatwith the top fraction of the distillation unit.

A twenty first aspect of the present invention provides a system (1) forthe liquefaction of natural gas that cools the natural gas to produceliquefied natural gas, comprising: a first expander (3) for generatingpower by expanding natural gas under pressure as material gas; adistillation unit (15) for reducing or eliminating a heavy component inthe material gas by distilling the material gas depressurized byexpansion in the first expander; a first compressor (4) for compressingthe material gas from which the heavy component was reduced oreliminated by the distillation unit by using power generated in thefirst expander; and a liquefaction unit (21) for liquefying the materialgas compressed by the first compressor by exchanging heat with arefrigerant.

According to the twenty first aspect of the present invention, inconjunction with the liquefaction of material gas at a relatively highpressure (100 barA or higher, for instance), the power generated by thefirst expander owing to the expansion of the material gas can be usedfor increasing the outlet pressure of the first compressor.

A twenty second aspect of the present invention provides a system (1)for the liquefaction of natural gas that cools the natural gas toproduce liquefied natural gas, comprising: a first expander (3) forgenerating power by expanding natural gas under pressure as materialgas; a first cooling unit (10, 11, 12) for cooling the material gas atleast at a point upstream or downstream of the first expander; adistillation unit (15) for reducing or eliminating a heavy component inthe material gas by distilling the material gas cooled by the firstcooling unit; a first compressor (4) for compressing the material gasfrom which the heavy component was reduced or eliminated by thedistillation unit; and a liquefaction unit (21) for liquefying a gasphase component separated from the material gas compressed by the firstcompressor by exchanging heat with a refrigerant.

According to the twenty second aspect of the present invention, thematerial gas that is compressed by the compressor and introduced intothe liquefaction unit is prevented from rising excessively intemperature, and the temperature of the material gas can be adjusted tobe close the temperature at the inlet end of the liquefaction unit withease.

A twenty third aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a first gas-liquidseparation vessel (23) for receiving the material gas compressed by thefirst compressor and a second cooling unit (85) provided between thefirst compressor and the first gas-liquid separation vessel for coolingthe compressed gas expelled from the first compressor.

According to the twenty third aspect of the present invention, thematerial gas that is to be introduced into the first gas-liquidseparation vessel is not required to be cooled by the liquefaction unitso that the load on the liquefaction unit can be reduced.

A twenty fourth aspect of the present invention provides a system forthe liquefaction of natural gas, further comprising a second gas-liquidseparation vessel (25) for receiving a part of the compressed gas thatis compressed and separated by the first compressor, and a liquid phasecomponent separated by the second gas-liquid separation vessel isrecirculated to the distillation unit.

According to the twenty fourth aspect of the present invention, evenwhen the critical pressure of the material gas is relatively low, andthe pressure of the material gas that is to be processed by theliquefaction system is higher than the critical pressure, theliquefaction load of the liquefaction unit can be reduced, and theprocess stability of the distillation unit can be enhanced.

A twenty fifth aspect of the present invention provides a system for theliquefaction of natural gas, further comprising a heat exchanger (69)for exchanging heat between the material gas that is introduced into thedistillation unit and a top fraction from the distillation unit.

According to the twenty fifth aspect of the present invention, even whenthe temperature of the material gas that is introduced into theliquefaction unit is lower than an appropriate range, the temperature ofthe material gas can be brought close the temperature at the inlet endof the liquefaction unit (21) by warming the top fraction of thedistillation unit by exchanging heat with the material gas that is to beintroduced into the distillation unit.

A twenty sixth aspect of the present invention provides a method for theliquefaction of natural gas by cooling the natural gas to produceliquefied natural gas, comprising: a first expansion step for generatingpower by using natural gas under pressure as material gas; a firstcooling step for cooling the material gas depressurized by expansion inthe first expansion step; a distillation step for reducing oreliminating a heavy component in the material gas by distilling thematerial gas cooled in the first cooling step; and a first compressionstep for compressing the material gas from which the heavy component wasreduced or eliminated in the distillation step by using the powergenerated in the first expansion step; and a liquefaction step forliquefying the material gas compressed in the first compression step byexchanging heat with a refrigerant.

A twenty seventh aspect of the present invention provides a method forthe liquefaction of natural gas by cooling the natural gas to produceliquefied natural gas, comprising: a first expansion step for generatingpower by expanding natural gas under pressure as material gas; a firstcooling step for cooling the material gas at least before or after thefirst expansion step; a distillation step for reducing or eliminating aheavy component in the material gas by distilling the material gascooled in the first cooling step; a first compression step forcompressing the material gas from which the heavy component was reducedor eliminated in the distillation step; and a liquefaction step forliquefying a gas phase component separated from the material gascompressed in the first compression step by exchanging heat with arefrigerant.

Effect of the Invention

As can be appreciated from the foregoing, the liquefaction system forthe liquefaction of natural gas according to the present inventionallows the outlet pressure of the compressor to be increased by usingthe power generated by the expander owing to the expansion of thematerial gas, and the cooling capacity that is required for the coolingunit to be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first embodiment of thepresent invention;

FIG. 2 is a diagram showing a liquefaction process flow in aconventional system for the liquefaction of natural gas given as a firstexample for comparison;

FIG. 3 is a diagram showing a liquefaction process flow in aconventional system for the liquefaction of natural gas given as asecond example for comparison;

FIG. 4 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first modification of thefirst embodiment;

FIG. 5 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second modification of thefirst embodiment;

FIG. 6 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a third modification of thefirst embodiment;

FIG. 7 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fourth modification of thefirst embodiment;

FIG. 8 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fifth modification of thefirst embodiment;

FIG. 9 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a sixth modification of thefirst embodiment;

FIG. 10 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a seventh modification of thefirst embodiment;

FIG. 11 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second embodiment of thepresent invention;

FIG. 12 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a third embodiment of thepresent invention;

FIG. 13 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a modification of the thirdembodiment;

FIG. 14 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fourth embodiment of thepresent invention;

FIG. 15 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fifth embodiment of thepresent invention;

FIG. 16 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a sixth embodiment of thepresent invention;

FIG. 17 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first modification of thesixth embodiment;

FIG. 18 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second modification of thesixth embodiment;

FIG. 19 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a third modification of thesixth embodiment;

FIG. 20 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fourth modification of thesixth embodiment;

FIG. 21 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a seventh embodiment of thepresent invention;

FIG. 22 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as an eighth embodiment of thepresent invention;

FIG. 23 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first modification of theeighth embodiment;

FIG. 24 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second modification of theeighth embodiment;

FIG. 25 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a ninth embodiment of thepresent invention;

FIG. 26 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a modification of the ninthembodiment;

FIG. 27 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a tenth embodiment of thepresent invention;

FIG. 28 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first modification of thetenth embodiment;

FIG. 29 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second modification of thetenth embodiment;

FIG. 30 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as an eleventh embodiment of thepresent invention;

FIG. 31 is a diagram showing a first variation of the connectingarrangement between the expander and the compressor in the system forthe liquefaction of natural gas according to the present invention;

FIG. 32 is a diagram showing a second variation of the connectingarrangement between the expander and the compressor in the system forthe liquefaction of natural gas according to the present invention; and

FIG. 33 is a diagram showing the liquefaction flow in the system for theliquefaction of natural gas of an eighth modification of the firstembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Preferred embodiments of the present invention are described in thefollowing with reference to the appended drawings.

First Embodiment

FIG. 1 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first embodiment of thepresent invention. Table 1 which will be shown hereinafter lists theresults of a simulation of the liquefaction process in the system forthe liquefaction of natural gas. The same is similarly true with Tables2 to 12. Table 1 shows the temperature, the pressure, the flow rate andthe molar composition of the natural gas that is to be liquefied at eachof various points in the liquefaction system of the first embodiment. InTable 1, columns (i) to (ix) show the values at the respective points inthe liquefaction system 1 denoted with corresponding roman numerals (i)to (ix) in FIG. 1.

Natural gas containing about 80 to 98 mol % of methane is used as thematerial gas or the feedstock gas. The material gas also contains atleast C5+ hydrocarbons by at least 0.1 mol % or BTX (benzene, toluene,xylene) by at least 1 ppm mol as heavier contents. The contents of thematerial gas other than methane are shown in column (i) of Table 1. Theterm “material gas” as used in this specification is not necessarilyrequired to be in gaseous form, but may also be in liquid form accordingto various stages of liquefaction.

In this liquefaction system 1, the material gas is supplied to a waterremoval unit 2 via a line L1, and is freed from moisture in order toavoid troubles due to icing. The material gas supplied to the waterremoval unit 2 has a temperature of about 20 degrees Celsius, a pressureof about 5,830 kPaA and a flow rate of about 720,000 kg/hr. The waterremoval unit 2 may consist of towers filled with desiccant (such as amolecular sieve), and can reduce the water content of the material gasto less than 0.1 ppm mol. The water removal unit 2 may consist of anyother known unit which is capable of removing water from the materialgas below a desired level.

Although detailed discussion is omitted here, the liquefaction system 1may employ additional known facilities for performing preliminaryprocess steps preceding the process step in the water removal unit 2,such as a separation unit for removing natural gas condensate, an acidgas removal unit for removing acid gases such as carbon dioxide andhydrogen sulfide and a mercury removal unit for removing mercury.Typically, the water removal unit 2 receives material gas from whichimpurities are removed by using such facilities. The material gas thatis supplied to the water removal unit 2 is pre-processed such that thecarbon dioxide (CO₂)content is less than 50 ppm mol, the hydrogensulfide (H₂S)content is less than 4 ppm mol, the sulfur content is lessthan 20 mg/Nm³, and the mercury content is less than 10 ng/Nm³.

The source of the material gas may not be limited to any particularsource, but may be obtained, not exclusively, from shale gas, tight sandgas and coal head methane in a pressurized state. The material gas maybe supplied not only from the source such as a gas field via piping butalso from storage tanks.

The material gas from which water is removed in the water removal unit 2is forwarded to a first expander 3 via a line L2. The first expander 3consists of a turbine for reducing the pressure of the natural gassupplied thereto, and obtaining power (or energy) from the expansion ofthe natural gas under an isentropic condition. Owing to the expansionstep (first expansion step) in the first expander 3, the pressure andthe temperature of the material are reduced. The first expander 3 isprovided with a common shaft 5 to a first compressor 4 (which will bediscussed hereinafter) so that the power generated by the first expander3 can be used for powering the first compressor 4. If the rotationalspeed of the first expander 3 is lower than that of the first compressor4, a suitable step-up gear unit may be placed between the first expander3 and the first compressor 4. The first expander 3 reduces thetemperature and the pressure of the material gas to about 8.3 degreesCelsius, a pressure of about 4,850 kPaA, respectively. Typically, thepressure of the material gas expelled from the first expander 3 is inthe range of 3,000 kPaA to 5,500 kPaA (30 barA to 55 barA), or morepreferably in the range of 3,500 kPaA to 5,000 kPaA (35 barA to 50barA).

The material gas from the first expander 3 is forwarded a cooler 11 viaa line L3. A cooling unit (first cooling unit) is formed by connectinganother cooler 12 to the downstream end of the cooler 11. The materialgas is cooled by exchanging heat with refrigerants (first cooling step)in the first cooling unit 11, 12 in stages. The temperature of thematerial gas which has been cooled by the first cooling unit 11, 12 isin the range of from −20 to −50 degrees Celsius, or more preferably inthe range of from −25 to −35 degrees Celsius. If the material gasintroduced into the liquefaction system 1 is relatively high (higherthan 100 barA, for instance), the first cooling unit 11, 12 may beomitted as the temperature of the material gas at the outlet of thefirst expander 3 is relatively low (−30 degrees Celsius, for instance).The possibility of omitting the cooling unit on the upstream side of thedistillation unit 15 applies equally to the embodiments illustrated inFIGS. 4 to 26, 30 and 33 which will be discussed hereinafter.

In the present embodiment, the C3-MR (propane (C3) pre-cooled mixedrefrigerant) system is used. The material gas is pre-cooled in the firstcooling unit 11, 12 by using propane as the refrigerant, and is latersuper-cooled to an extremely low temperature for the liquefaction of thematerial gas in a refrigeration cycle using mixed refrigerants as willbe discussed hereinafter. Propane refrigerants (C3R) for medium pressure(MP) and low pressure (LP) are used for cooling the material gas in aplurality of stages (in two stages in the illustrated embodiment) in thefirst cooling unit 11, 12. Although not shown in the drawings, the firstcooling unit 11, 12 forms a part of a per se known refrigeration cycleincluding compressors and condensers for the propane refrigerants.

The liquefaction system 1 is not necessarily required to be based on theC3-MR system, but may use a cascade system in which a plurality ofindividual refrigeration cycles are formed by using correspondingrefrigerants (such as methane, ethane and propane) having differentboiling points, a DMR (double mixed refrigerant) system using a mixedmedium such as a mixture of ethane and propane for a preliminary coolingprocess, and a MFC (mixed fluid cascade system) using different mixedrefrigerants separately for the individual cycles of preliminarycooling, liquefaction and super cooling, among other possibilities.

The material gas from the cooler 12 is forwarded to the distillationunit 15 via a line L4. The pressure of the material gas at this pointshould be below the critical pressures of methane and heavier componentsby means of the expansion in the first expander 3 and other optionalprocesses. The distillation unit 15 essentially consists of adistillation tower internally provided with a plurality of shelves forremoving heavier contents in the material gas (distillation step). Theliquid consisting of the heavier contents is expelled via a line L5connected to the bottom end of the distillation tower of thedistillation unit 15. The liquid consisting of the heavier contents thatis expelled from the distillation unit 15 via the line L5 has atemperature of about 177 degrees Celsius and a flow rate of about 20,000kg/hr. The term “heavier contents” refer to components such as benzenehaving high freezing points and components having lower boiling pointssuch as C5+ hydrocarbons. The line L5 includes a recirculation unitincluding a reboiler 16 for heating a part of the liquid expelled fromthe bottom of the distillation tower of the distillation unit 15 byexchanging heat with steam (or oil) supplied to the reboiler 16 fromoutside, and recirculating the heated liquid back to the distillationunit 15.

The top fraction from the distillation unit 15 consisting of the lightercomponents of the material gas primarily consists of methane having alow boiling point, and this material gas is introduced into theliquefaction unit 21 via the line L6 to be cooled in the piping systems22 a and 22 b. The material gas forwarded to the line L5 has atemperature of about −45.6 degrees Celsius and a pressure of about 4,700kPaA. The material gas freed from the heavier components in thedistillation unit 15 contains less than 0.1 mol % of C5+ and less than 1ppm mol of BTX (benzene, toluene and xylene). By flowing through thepiping systems 22 a and 22 b, the material gas is cooled to about −65.2degrees Celsius, and is then forwarded from the liquefaction unit 21 toa first gas-liquid separation vessel 23 via a line L7.

As will be discussed hereinafter, the liquefaction unit 21 essentiallyconsists of a main heat exchanger in the liquefaction system 1, and thisheat exchanger consists of a spool-wound type heat exchanger including ashell and coils of heat transfer tubes for conducting the material gasand the refrigerant. The liquefaction unit 21 defines a warm region Z1situated in the lower part thereof for receiving the mixed refrigerantand having a highest temperature (range), an intermediate region Z2situated in the intermediate part thereof and having a lower temperaturethan the warm region Z1 and a cold region situated in the upper partthereof for expelling the liquefied material gas and having a lowesttemperature. In the first embodiment, the warm region Z1 consists of ahigher warm region Z1 a on a higher temperature side and a lower warmregion Z1 b on a lower temperature side. The piping systems 22 a and 22b, as well as the piping systems 42 a, 51 a, and 42 b and 51 b throughwhich the mixed refrigerant is conducted, are formed by the tube bundlesprovided in the higher warm region Z1 a and the lower warm region Z1 b,respectively. In the illustrated embodiment, the temperature of thehigher warm region Z1 a is about −35 degrees Celsius on the upstreamside (inlet side) of the material gas that is to be cooled, and about−50 degrees Celsius on the downstream side (outlet side) of the materialgas. The temperature of the lower warm region Z1 b is about −50 degreesCelsius on the upstream side of the material gas, and about −135 degreesCelsius on the downstream side of the material gas. The temperature ofthe intermediate region Z2 is about −65 degrees Celsius on the upstreamside of the material gas, and about −135 degrees Celsius on thedownstream side of the material gas. The temperature of the cold regionZ3 is about −135 degrees Celsius on the upstream side of the materialgas, and about −155 degrees Celsius on the downstream side of thematerial gas. The temperatures on the upstream side and the downstreamside of each region are not limited to the values mentioned here, andthe temperature in each of these parts may vary within a prescribedrange (±5 degrees Celsius, for instance).

The first gas-liquid separation vessel 23 separates the liquid phasecomponent (condensate) of the material gas, and this liquid essentiallyconsisting of hydrocarbons is recirculated back to the distillation unit15 by a recirculation pump 24 provided in a line L8. The gas phasecomponent of the material gas obtained in the first gas-liquidseparation vessel 23 and mainly consisting of methane is forwarded to afirst compressor 4 via a line L9. The material gas is passed through theline L8 at a flow rate of about 83,500 kg/hr, and is passed through theline L6 at a flow rate of about 780,000 kg/hr. The first gas-liquidseparation vessel 23 may also be cooled by using a mixed refrigerant oran ethylene refrigerant.

The first compressor 4 consists of a single stage centrifugal compressorhaving turbine blades for compressing the material gas, mounted on ashaft 5 common to the first expander 3. The material gas compressed bythe first compressor 4 (first compression step) is introduced into theliquefaction unit 21 via a line L10. The material gas that is put out bythe first compressor 4 to the line L10 has a temperature of about −51degrees Celsius and a pressure of about 5,500 kPaA. The material gasintroduced into the liquefaction unit 21 is compressed by the firstcompressor 4 preferably to a pressure exceeding at least 5,171 kPaA.

A line L10 is connected to a piping system 30 positioned in the warmregion Z1 b of the liquefaction unit 21, and the upstream end of thispiping system 30 is connected to a piping system 31 in the intermediateregion Z2, and then to a piping system 32 positioned in the cold regionZ3. After being liquefied and super cooled by flowing through the pipingsystems 31 and 32, the natural gas is forwarded to an LNG tank forstorage purpose not shown in the drawings via an expansion valve 33provided in a line L11. The material gas subjected to the liquefactionstep acquires a temperature of −162 degrees Celsius and a pressure ofabout 120 kPaA in the downstream end of the expansion valve 33.

The material gas flowing through the liquefaction unit 21 is cooled by arefrigeration cycle using mixed refrigerants. In the illustratedembodiment, the mixed refrigerants may each contain nitrogen in additionto a mixture of hydrocarbons including methane, ethane and propane, butmay also have other per se known compositions as long as the requiredcooling capability can be achieved.

In the liquefaction unit 21, a high pressure (HP) mixed refrigerant (MR)is supplied to a refrigerant separator 41 via a line L12. The mixedrefrigerant which makes up the liquid phase component in the refrigerantseparator 41 is introduced into the liquefaction unit 21 via a line L13,and then flows upward in the liquefaction unit 21 through the pipingsystems 42 a and 42 b positioned in the warm regions Zla and Z1 b,respectively, and the piping system 43 positioned in the intermediateregion Z2. The mixed refrigerant is then expanded in an expansion valve44 provided in a line L14, and is partly flash vaporized.

After passing through the expansion valve 44, the mixed refrigerant isejected downward (so as to oppose the flow of the material gas in theliquefaction unit 21) from a spray header 45 provided in an upper partof the intermediate region Z2. The mixed refrigerant ejected from thespray header 45 flows downward while exchanging heat with anintermediate tube bundle formed by the piping systems 31, 43 and 52 (thelast piping system will be discussed hereinafter) positioned in theintermediate region Z2, and a lower tube bundle formed by the pipingsystems 22 a, 22 b, 30, 42 a, 42 b, 51 a and 51 b (the last two pipingsystems will be discussed hereinafter) positioned in the warm region Z1.

The mixed refrigerant that makes up the gas phase of the refrigerantseparator 41 is introduced into the liquefaction unit 21 via a line L15,and then flows upward in the liquefaction unit 21 by flowing through thepiping systems 51 a and 51 b positioned in the warm regions Z1 a and Z1b, the piping system 52 in the intermediate region Z2 and the pipingsystem 53 positioned in the cold region Z3. The mixed refrigerant isthen expanded in an expansion valve 54 provided in a line L16, and ispartly flash vaporized.

The mixed refrigerant that has passed through the expansion valve 54 isalready cooled to a temperature below the boiling point of methane(about −167 degrees Celsius in this case), and is expelled downward froma spray header 55 positioned in an upper part of the cold region Z3 (orflows in opposite direction to the flow of the material gas in theliquefaction unit 21). The mixed refrigerant ejected from the sprayheader 55 flows downward while exchanging heat with an upper tube bundleformed by the piping systems 32 and 53 positioned in the cold region Z3,and after mixing with the mixed refrigerant ejected from the sprayheader 45 located below, flows downward while exchanging heat with theintermediate tube bundle formed by the piping systems 31, 43 and 52positioned in the intermediate region Z2, and the lower tube bundleformed by the piping systems 22 a, 22 b, 30, 42 a, 42 b, 51 a and 51 bpositioned in the warm region Z1.

The mixed refrigerant ejected from the spray headers 45 and 55 isfinally expelled via a line L17 connected to the bottom end of theliquefaction unit 21 as low pressure (LP) mixed refrigerant (MP) gas.The facilities for the mixed refrigerant provided in the liquefactionunit 21 (such as the refrigerant separator 41) form a part of a per seknown refrigeration cycle for the mixed refrigerant, and the mixedrefrigerant put out to the line L17 is recirculated to the refrigerantseparator 41 via the line L12 after passing through compressors andcondensers.

As discussed above, the material gas introduced into the liquefactionsystem 1 is effectively liquefied after being processed in the expansionstep, the cooling step, the distillation step, the compression step andthe liquefaction step. This liquefaction system can be applied, forinstance, to a base load liquefaction plant for producing liquefiednatural gas (LNG) mainly consisting of methane from the material gasmined from a gas field.

TABLE 1 No. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) vapor phasefaction 1.00 1.00 1.00 0.00 0.93 0.00 1.00 1.00 0.00 temperature [C.]20.08 8.32 −42.58 177.19 −65.24 −65.24 −65.24 −50.99 −161.55 pressure[kPa] 5830.00 4850.00 4700.00 4705.00 4400.00 4400.00 4400.00 5483.00120.00 molar flow rate 42000 42000 45020 313 45020 3334 41686 4168641700 [kgmole/h] mass flow rate [kg/h] 719619 719619 783504 19764 78350483548 699948 699948 698733 molar fraction nitrogen 0.0081995900.000033626 0.008260844 methane 0.949922502 0.043508871 0.956667221ethane 0.024998750 0.032339550 0.024931118 propane 0.0099995000.143654595 0.009078200 bentane 0.001999900 0.165149865 0.000793571n-butane 0.001999900 0.232835468 0.000268518 i-pentane 0.0004999750.066891833 0.000001710 n-pentane 0.000499975 0.067093928 0.000000817n-bexane 0.000599970 0.080591895 0.000000000 benzene 0.0004999750.067159786 0.000000000 toluene 0.000099995 0.013432078 0.000000000p-xylene 0.000049998 0.006716040 0.000000000 n-heptane 0.0004999750.067160391 0.000000000 n-octane 0.000099995 0.013432079 0.000000000

(First and Second Examples for Comparison)

FIGS. 2 and 3 are diagrams showing liquefaction process flows inconventional systems for the liquefaction of natural gas given as afirst and a second example for comparison with the first embodiment ofthe present invention. In the conventional liquefaction systems 101 and201 for natural gas, the parts corresponding to those of theliquefaction system of the first embodiment are denoted with likenumerals. Tables 2 and 3 show the temperature, pressure, flow rate andmolar fractions of the material gas in the liquefaction systems of thefirst and second examples for comparison, respectively. It should benoted that the liquefaction system 201 of the second example forcomparison is based on the prior art disclosed in Patent Document 1(U.S. Pat. No. 4,065,278).

As shown in FIG. 2, the liquefaction system 101 of the first example forcomparison is not provided with the first expander 3 and the firstcompressor 4 used in the liquefaction system 1 of the first embodiment,and the material gas expelled from the water removal unit 2 is forwardedto a cooler 110 via a line L101. A cooler unit is formed by connecting acooler 11 and a cooler 12 to the downstream end of the cooler 110 in aserial connection so that the material gas is sequentially cooled byexchanging heat in the three coolers 110, 11 and 12 which use a highpressure (HP), a medium pressure (MP) and a low pressure (LP) propanerefrigerant, respectively. The material gas expelled from the cooler 12in the downstream end has a temperature of about −34.5 degrees Celsiusand a pressure of about 5,680 kPaA. The material gas is thendepressurized by an expansion in an expansion valve 113 in a line L4,and is then introduced into a distillation unit 15.

In the liquefaction system 101, the material gas forming a gas phasecomponent in the first gas-liquid separation vessel 23 and essentiallyconsisting of methane is introduced into the piping system 31 positionedin the intermediate region Z2 of the liquefaction unit 21 via a lineL102. The material gas that is put out from the first gas-liquidseparation vessel 23 to a line L12 has a temperature of about −65.3degrees Celsius and a pressure of about 4,400 kPaA.

TABLE 2 No. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) vapor phasefaction 1.00 0.99 1.00 0.00 0.93 0.00 1.00 0.00 temperature [C.] 20.08−34.50 −42.58 176.73 −65.25 −65.25 −65.25 −161.56 pressure [kPa] 5830.005680.00 4700.00 4705.00 4400.00 4400.00 4400.00 120.00 molar flow rate[kgmole

42000 42000 45020 314 45020 3334 41686 41700 mass flow rate [kg/h]719619 719619 783488 19624 783454 83495 699951 696348 molar fractionnitrogen 0.008199590 0.000072318 0.008260784 methane 0.9499525020.064051796 0.956622861 ethane 0.024998750 0.031841875 0.024947225propane 0.009999500 0.129428030 0.009100267 butane 0.0019999000.161816482 0.000796567 n-butane 0.001999900 0.231738008 0.000270095i-pentane 0.000499975 0.066667173 0.000001771 n-pentane 0.0004999750.066846201 0.000000423 n-hexane 0.000599970 0.080282498 0.000000003benzene 0.000499975 0.066901980 0.000000003 toluene 0.0000999950.013380485 0.000000000 p-xylene 0.000049998 0.006690243 0.000000000n-heptane 0.000499975 0.066902427 0.000000000 n-octane 0.0000999950.013380486 0.000000000

indicates data missing or illegible when filed

As shown in FIG. 3, the liquefaction system 201 of the second examplefor comparison is an improvement of the liquefaction system 101 of thefirst example for comparison, and is provided with a first expander 3and a first compressor 4. However, as opposed to the first expander 3used in the liquefaction system 1 of the first embodiment, the expander3 is positioned on the downstream side of the cooling unit (consistingthe three coolers 110, 11 and 12 in this case). In the liquefactionsystem 201, the material gas expelled from the cooler 12 is forwarded toa separator 213 to be separated into gas and liquid components. Thematerial gas that forms the gas phase component in the separator 213 isforwarded to the expander 3 to be expanded therein, and is thenforwarded to the distillation unit 15 via a line L204. The part of thematerial gas that forms the liquid component in the separator 213 is putout to a line L205 provided with an expansion valve 214. The liquid thathas been expanded in the expansion valve 214 is then forwarded to thedistillation unit 15 via the line L204 along with the material gas fromthe expander 3.

The liquefaction system 201 is similar to that of the first embodimentas far as the part thereof downstream of the distillation unit 15 isconcerned, and the material gas that has been put out to the line L10 bythe compressor 4 has a temperature of about −54.7 degrees Celsius and apressure of about 5,120 kPaA.

TABLE 3 No. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) vapor phase1.00 1.00 1.00 0.00 0.94 0.00 1.00 1.00 0.00 faction temperature 20.08−45.36 −44.83 208.13 −64.56 −64.56 −64.56 −54.74 −161.59 [C.] pressure[kPa] 5830.00 4705.00 4700.00 4705.00 4400.00 4400.00 4400.00 5120.00120.00 molar flow 42000 41783 44200 302 44200 2500 41700 41700 41700rate [kg

mass flow 719619 709009 764342 19107 764342 63861 700471 700471 694674rate [kg/

molar fraction nitrogen 0.008199590 0.000051871 0.008259333 methane0.949952502 0.053398407 0.956509212 ethane 0.024998750 0.0320759320.024927984 propane 0.009999500 0.133750785 0.009066826 butane0.001999900 0.153843084 0.000893180 n-butane 0.001999900 0.2308052330.000340430 i-pentane 0.000499975 0.069219794 0.000002448 n-pentane0.000499975 0.069480324 0.000000589 n-hexane 0.000599970 0.0834726420.000000000 benzene 0.000499975 0.069560398 0.000000000 toluene0.000099995 0.013912204 0.000000000 p-xylene 0.000049998 0.0069561020.000000000 n-heptane 0.000499975 0.069561020 0.000000000 n-octane0.000099995 0.013912205 0.000000000

indicates data missing or illegible when filed

As can be appreciated by comparing the first and second examples forcomparison with the present invention, the liquefaction system 1according to the present invention allows a greater power to be producedby expanding material gas of higher temperature and higher pressurebecause the first expander 3 is positioned on the upstream side of thefirst cooling unit 11, 12, as compared to the liquefaction system 201 ofthe second example which has the expander 3 positioned on the downstreamside of the cooling unit 110, 11, 12. As a result, the first compressor4 can be driven with an increased power (or the outlet pressure of thefirst compressor 4 can be increased) so that the pressure of thematerial gas introduced into the liquefaction unit 21 can be increased,and the efficiency of the liquefaction process in the liquefaction unit21 can be advantageously increased.

The liquefaction system 1 of the illustrated embodiment provides anadditional advantage of reducing the required cooling capacity of thecooling unit (thereby allowing the cooler 110 in the second example forcomparison to be omitted) because the temperature of the material gas isreduced by the expansion of the material gas in the first expander 3owing to the positioning of the first expander 3 on the upstream side ofthe first cooling unit 11, 12. In the liquefaction system 1 of theillustrated embodiment, the gas-liquid separation vessel (separator 213)for removing the condensate of the material gas placed between thecooling unit and the expander 3 may be omitted.

First Modification of the First Embodiment

FIG. 4 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a first modification of thefirst embodiment. In the liquefaction system illustrated in FIG. 4, theparts corresponding to those of the liquefaction system 1 of the firstembodiment are denoted with like numerals and omitted from the followingdiscussion except for the matters that will be discussed in thefollowing.

In the liquefaction system of the first embodiment, a cascaderefrigeration system using methane and ethylene for refrigerants isemployed. The main heat exchanger is formed by a methane heat exchanger21 a and an ethylene heat exchanger 21 b each consisting of a plate-fintype heat exchanger, instead of the spool-wound heat exchanger(liquefaction unit 21) of the first embodiment.

The methane heat exchanger 21 a defines a warm region having a firstheat transfer unit 61 that receives a high pressure (HP) methanerefrigerant (C1R), an intermediate region having a second heat transferunit 62 that receives a medium pressure (MP) methane refrigerant and acold region having a third heat transfer unit 63 that receives a lowpressure (LP) methane refrigerant.

The ethylene heat exchanger 21 b defines a warm region having a fourthheat transfer unit 64 that receives a high pressure (HP) ethylenerefrigerant (C2R), an intermediate region having a fifth heat transferunit 65 that receives a medium pressure (MP) ethylene refrigerant and acold region having a sixth heat transfer unit 66 that receives a lowpressure (LP) ethylene refrigerant.

The material gas that is separated as the top fraction in thedistillation unit 15 is introduced into the liquefaction unit 21 via theline L6, and is cooled by a seventh heat transfer unit 22 positionedover the warm region and the intermediate region in the ethylene heatexchanger 21 b. The material gas compressed by the first compressor 4 isforwarded to the ethylene heat exchanger 21 b via the line L10. Thematerial gas that flows the line L10 is introduced into an eighth heattransfer unit 67 positioned over the intermediate region and the coldregion of the ethylene heat exchanger 21 b in two stages. The materialgas expelled from the ethylene heat exchanger 21 b is introduced into aninth heat transfer unit 68 extending from the warm region to the coldregion of the ethane heat exchanger 21 a to be cooled in the warmregion, the intermediate region and the cold region in three stages.

In the liquefaction system 1 of the first modification of the firstembodiment of the present invention, an advantage in the facility ofchanging the point of connecting the line L10 to the main heat exchanger(the point of introducing the material gas into the ethylene heatexchanger 21 b) can be gained owing to the use of the plate-fin heatexchanger as the main heat exchanger. Therefore, even when thetemperature level of the material gas flowing through the line L10 risesalong with the pressure thereof, by changing the point of introducingthe material gas into the heat exchanger depending on the temperaturelevel of the material gas (or by bringing the temperature of thematerial close to the temperature at the point of introduction into theheat exchanger), the thermal load on the heat exchanger can be reduced,and the efficiency of the liquefaction process can be increased.

Second, Third and Fourth Modifications of the First Embodiment

FIGS. 5, 6 and 7 are diagrams showing liquefaction process flows insystems for the liquefaction of natural gas given as a second, third andfourth modifications of the first embodiment, respectively. In theliquefaction systems illustrated in FIGS. 5, 6 and 7, the partscorresponding to those of the liquefaction system 1 of the firstembodiment (as well as the other modifications) are denoted with likenumerals and omitted from the following discussion except for thematters that will be discussed in the following.

As shown in FIG. 5, in the liquefaction system 1 of the secondmodification, a heat exchanger 69 is provided between the line L4 andthe line L9. Thus, the material gas that is separated in the firstgas-liquid separation vessel 23 as the gas phase component and flowsthrough the line L9 is heated by exchanging heat with the material gasflowing from the cooling unit 12 to the distillation unit 15 via theline L4, before being introduced into the first compressor 4. Thematerial gas compressed by the first compressor 4 is introduced into theliquefaction unit 21 via the line L10. The downstream end of the lineL10 is connected to a piping system 30 positioned in the warm region Z1demonstrating the highest temperature in the liquefaction unit 21. Thepiping system 30 forms a tube bundle that is positioned in the warmregion Z1 jointly with a piping system 22 into which the top fraction ofthe distillation unit 15 is introduced, and a piping system 42 and apiping system 51 through which a mixed refrigerant flows.

Owing to this arrangement, in the second modification of the firstembodiment, even when the temperature level of the material gas that isintroduced into the liquefaction unit 21 via the line L10 should belower than an appropriate range, the temperature of the material gas canbe raised to an appropriate level by exchanging heat in the heatexchanger 69. In other words, in the second modification of the firstembodiment, the temperature of the material gas in the line L10 afterthe compression can be brought close to the temperature at the point ofintroduction (piping system 30) in the liquefaction unit 21 (preferablywith a deviation of less than 10 degrees Celsius) so that the thermalload on the liquefaction unit 21 can be reduced (or the generation ofthermal stress can be minimized).

The arrangement of the heat exchanger 69 in the second modification canbe freely changed as long as the temperature of the material gas in theline L10 after the compression can be brought close to the temperatureat the introduction point of the liquefaction unit 21. For instance, inthe liquefaction system 1 of the third modification shown in FIG. 6, theheat exchanger 69 is provided between the line L4 and the line L10. Thematerial gas compressed by the first compressor 4 and flowing throughthe line L10 is cooled by exchanging heat with the material gas flowingthrough the line L4 before being introduced into the liquefaction unit21. In the third modification, because the material gas heated by theheat exchanger 69 is introduced into the liquefaction unit 21 withoutthe intervention of a device such as the first compressor 4, thetemperature of the material gas that is introduced into the liquefactionunit 1 can be controlled with ease.

As shown in FIG. 7, in the liquefaction system of the fourthmodification, the heat exchanger 69 is provided between the line L4 andthe line L6. Therefore, the material gas that is separated as a topfraction from the distillation unit 15 and flows through the line L6 isheated by exchanging heat with the material gas flowing through the lineL4, before being introduced into the liquefaction unit 21 (the pipingsystem 22). In particular, in the fourth modification, even when thematerial gas consists of natural gas (lean gas) containing a relativelylow level of heavier components (higher hydrocarbons) as shown in Table1, and the temperature of the material gas flowing through the line L6following the distillation step may fall below an appropriate range, thetemperature of the material gas can be raised to an appropriate level byexchanging heat in the heat exchanger 69.

Fifth Modification of the First Embodiment

FIG. 8 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fifth modification of thefirst embodiment. In the liquefaction system illustrated in FIG. 8, theparts corresponding to those of the liquefaction system 1 of the firstembodiment (including the modifications thereof) are denoted with likenumerals and omitted from the following discussion except for thematters that will be discussed in the following.

The fifth modification is similar to the fourth modification, butfurther includes a heat exchanger 79 provided between the line L9 andthe line L10. A fifth cooler 80 using a low pressure (LP) propanerefrigerant (C3R) is further provided in the line L10. As a result, thematerial gas expelled from the first compressor 4 is cooled byexchanging heat with the material gas flowing through the line L9 beforebeing introduced into the liquefaction unit 21. The downstream end ofthe line L10 is connected to a piping system 31 positioned in theintermediate region Z2.

In the fifth modification, the material gas expelled from the firstcompressor 4 can be introduced into the intermediate region Z2.Therefore, the tube bundle in the warm region Z1 can be formed by thethree piping systems 22, 42 and 51, and the tube bundle in theintermediate region Z2 can be formed by the three piping systems 31, 43and 52. As a result, in the fifth modification, when the liquefactionunit 21 is formed by using a spool-wound heat exchanger, the arrangementof the piping systems in the warm region Z1 and the intermediate regionZ2 can be optimized (by uniformly spreading the piping systems among thedifferent regions) as compared to the arrangement of the fourthmodification so that the size of the liquefaction unit 21 is preventedfrom becoming excessively great. The fifth cooler 80 uses a propanerefrigerant similarly to the first and second coolers 11 and 12 in theillustrated embodiment, by may also use other forms of air-cooled orwater-cooled coolers.

Sixth Modification of the First Embodiment

FIG. 9 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a sixth modification of thefirst embodiment of the present invention. Table 4 shows thetemperature, the pressure, the flow rate and the molar composition ofthe natural gas that is to be liquefied at each of various points in theliquefaction system of the sixth modification by way of an example Table5 shows the temperature, the pressure, the flow rate and the compositionof the refrigerant in the refrigeration cycle of the mixed refrigerantused in the liquefaction system by way of an example. In theliquefaction system illustrated in FIG. 9, the parts corresponding tothose of the liquefaction system 1 of the first embodiment (includingthe modifications thereof) are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

As shown in FIG. 9, the liquefaction system 1 of the sixth modificationis similar to those of the second to the fourth modifications except forthe difference in the material gas composition and the presence (or theabsence) of the heat exchanger 69. The line L10 in this case isconnected to the piping system 31 positioned in the intermediate regionZ2 of the liquefaction unit 21. FIG. 9 also shows the structure of arefrigeration cycle system 70 using mixed refrigerants provided in theliquefaction system 1. The material gas in this case consists of naturalgas (rich gas) having a relatively high levels of heavier contents(higher hydrocarbons) as shown in Table 4. By appropriately adjustingthe expansion of the material gas in the first expander 3, the topfraction of the distillation unit 15 has a relatively low pressure(about 3,300 kPaA) as compared with the first embodiment. As a result,in comparison with the liquefaction process of the lean gas such as theone discussed in conjunction with the first embodiment, the natural gasliquid can be recovered at a relatively high efficiency (about 89% ofpropane and about 100% of butane, for instance) via the line L5connected to the bottom end of the distillation unit 15.

In the refrigeration cycle system 70, the mixed refrigerant of arelatively low pressure (about 320 kPaA) expelled from the liquefactionunit 21 via the line L17 is compressed (first stage) by a firstrefrigerant compressor 17, cooled by a first intercooler 27, compressed(second stage) by a second refrigerant compressor 18, cooled by a secondintercooler 28, compressed (third stage) by a third refrigerantcompressor 19, and cooled by a third intercooler 29. The mixedrefrigerant is then further cooled by a series of coolers including thefirst to fourth refrigerant coolers 34 to 37, and is introduced into arefrigerant separator 41 via the line L12. The first to fourthrefrigerant coolers 34 to 37 cool the mixed refrigerant by stages byexchanging heat with the super high pressure (HHP), high pressure (HP),medium pressure (MP) and low pressure (LP) propane refrigerants.

As discussed above, the refrigeration cycle system 70 is provided withpropane pre-cooling facilities (not shown in the drawings) for coolingthe material gas before being introduced into the liquefaction unit 21,and a propane refrigerant is used for this purpose. Such a refrigerationcycle system 70 can also be applied to the other embodiments (includingthe modifications thereof).

TABLE 4 No. (i) (ii) (iii) (iv) (v) vapor phase faction 1.00 0.97 1.000.00 0.86 temperature [C.] 20.0 −15.2 −49.3 101.7 −71.6 pressure [kPa]7000 3470 3300 3310 3000 molar flow rate 42000 42000 45704 2601 45704[kgmole/h] mass flow rate 803679 803679 822638 134105 822638 [kg/h]molar fraction nitrogen 0.001000 0.001000 0.000940 0.000000 0.000940methane 0.877900 0.877900 0.880344 0.001278 0.880344 ethane 0.0609000.060900 0.098820 0.084051 0.098820 propane 0.033600 0.033600 0.0198560.485204 0.019856 butane 0.006500 0.006500 0.000034 0.104921 0.000034n-butane 0.011500 0.011500 0.000007 0.185684 0.000007 i-pentane 0.0034000.003400 0.000000 0.054899 0.000000 n-pentane 0.002100 0.002100 0.0000000.033908 0.000000 n-hexane 0.003100 0.003100 0.000000 0.050055 0.000000benzene 0.000000 0.000000 0.000000 0.000000 0.000000 toluene 0.0000000.000000 0.000000 0.000000 0.000000 p-xylene 0.000000 0.000000 0.0000000.000000 0.000000 n-heptane 0.000000 0.000000 0.000000 0.000000 0.000000n-octane 0.000000 0.000000 0.000000 0.000000 0.000000 No. (vi) (vii)(viii) (ix) vapor phase faction 0.00 1.00 1.00 0.00 temperature [C.]−71.6 −71.6 −27.1 −159.0 pressure [kPa] 3000 3000 5752 120 molar flowrate 6306 39399 39399 39399 [kgmole/h] mass flow rate 153064 669574669574 669574 [kg/h] molar fraction nitrogen 0.000152 0.001066 0.0010660.001066 methane 0.533997 0.935775 0.935775 0.935775 ethane 0.3453010.059372 0.059372 0.059372 propane 0.120269 0.003785 0.003785 0.003785butane 0.000231 0.000002 0.000002 0.000002 n-butane 0.000051 0.0000000.000000 0.000000 i-pentane 0.000000 0.000000 0.000000 0.000000n-pentane 0.000000 0.000000 0.000000 0.000000 n-hexane 0.000000 0.0000000.000000 0.000000 benzene 0.000000 0.000000 0.000000 0.000000 toluene0.000000 0.000000 0.000000 0.000000 p-xylene 0.000000 0.000000 0.0000000.000000 n-heptane 0.000000 0.000000 0.000000 0.000000 n-octane 0.0000000.000000 0.000000 0.000000

TABLE 5 No. (xi) (xii) (xiii) (xiv) (xv) (xvi) (xvii) (xviii) vaporphase faction 0.29 1.00 0.00 0.00 0.00 0.00 0.00 1.00 temperature [C.]−34.5 −34.5 −34.5 −135.0 −139.5 −160.9 −167.0 −37.0 pressure [kPa] 59505950 5950 5020 365 4570 375 320 molar flow rate 64912 18845 46067 4606746067 18845 18845 64912 [kgmole/h] mass flow rate[kg/h] 1688828 4009271287901 1287901 1287901 400927 400927 1688828 molar fraction nitrogen0.095000 0.208834 0.048433 0.095000 methane 0.445000 0.625994 0.3709590.445000 ethane 0.290000 0.135564 0.353177 0.290000 propane 0.1700000.029607 0.227432 0.170000 butane 0.000000 0.000000 0.000000 0.000000n-butane 0.000000 0.000000 0.000000 0.000000 i-pentane 0.000000 0.0000000.000000 0.000000 n-pentane 0.000000 0.000000 0.000000 0.000000 n-hexane0.000000 0.000000 0.000000 0.000000 benzene 0.000000 0.000000 0.0000000.000000 toluene 0.000000 0.000000 0.000000 0.000000 p-xylene 0.0000000.000000 0.000000 0.000000 n-heptane 0.000000 0.000000 0.000000 0.000000n-octane 0.000000 0.000000 0.000000 0.000000

Seventh Modification of the First Embodiment

FIG. 10 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a seventh modification of thefirst embodiment of the present invention. Table 6 shows thetemperature, the pressure, the flow rate and the molar composition ofthe natural gas that is to be liquefied at each of various points in theliquefaction system of the seventh modification by way of an example. Inthe liquefaction system illustrated in FIG. 10, the parts correspondingto those of the liquefaction system 1 of the first embodiment (includingthe modifications thereof) are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

In the seventh modification, rich gas is used as the material gassimilarly as in the sixth modification, and this modification isadvantageous when the material gas is composed such that the criticalpressure thereof is relatively high. In the liquefaction system 1, athird cooler 86 using a low pressure (LP) propane refrigerant (C3R) isprovided in a line L6 connecting the distillation unit 15 to the firstgas-liquid separation vessel 23, and a second cooler 85 using a similarlow pressure propane refrigerant is provided in a line L10 connectingthe first compressor 4 to the liquefaction unit 21. Thus, the materialgas expelled from the distillation unit 15 to the line L6 is cooled bythe third cooler 86, and is introduced into the first gas-liquidseparation vessel 23. Therefore, in the seventh modification, thematerial gas to be introduced into the first gas-liquid separationvessel 23 is not required to be cooled by the liquefaction unit 21(piping system 22) as opposed to the other modifications such as thesixth modification so that the load on the liquefaction process of theliquefaction unit 21 can be reduced.

The material gas that is expelled from the first compressor 4 to theline L10 is cooled by the second cooler 85, and is then introduced intothe liquefaction unit 21. In this case, the downstream end of the lineL10 is connected to the piping system 30 which is positioned in the warmregion Z1 or the warmest part of the liquefaction unit 21. Thus, in theseventh modification, even when the temperature level of the materialgas should exceed an appropriate range owing to the compression of thematerial gas, the cooling in the second cooler 85 can bring thetemperature of the material gas close to the temperature level of thewarm region Z1 of the liquefaction unit 21 so that the thermal load(thermal stresses) on the liquefaction unit 21 can be reduced.

TABLE 6 No. (i) (ii) (iii) (iv) (v) vapor phase faction 1.00 1.00 1.000.00 0.96 temperature [C.] 20.0 10.3 −19.6 79.8 −19.6 pressure [kPa]8000 6830 6670 6680 6670 molar flow rate [kgm

42000 42000 41945 1822 41945 mass flow rate [kg/h] 807998 807998 77599283599 775992 molar fraction nitrogen 0.007000 0.007000 0.007086 0.0000250.007086 methane 0.871400 0.871400 0.886689 0.208770 0.886689 ethane0.060900 0.060900 0.060265 0.147847 0.060265 propane 0.033600 0.0336000.030817 0.220181 0.030817 butane 0.006500 0.006500 0.005176 0.0730710.005176 n-butane 0.011500 0.011500 0.008181 0.156499 0.008181 i-pentane0.003400 0.003400 0.001290 0.066143 0.001290 n-pentane 0.002100 0.0021000.000472 0.044574 0.000472 n-hexane 0.003100 0.003100 0.000021 0.0713760.000021 benzene 0.000500 0.000500 0.000003 0.011515 0.000003 toluene0.000000 0.000000 0.000000 0.000000 0.000000 p-xylene 0.000000 0.0000000.000000 0.000000 0.000000 n-heptane 0.000000 0.000000 0.000000 0.0000000.000000 n-octane 0.000000 0.000000 0.000000 0.000000 0.000000 No. (vi)(vii) (viii) (ix) vapor phase faction 0.00 1.00 1.00 0.00 temperature[C.] −32.6 −32.6 −34.5 −160.9 pressure [kPa] 6600 6600 7601 120 molarflow rate [kgm

1767 40178 40178 40178 mass flow rate [kg/h] 51592 724400 724400 724400molar fraction nitrogen 0.001845 0.007316 0.007316 0.007316 methane0.551125 0.901446 0.901446 0.901446 ethane 0.135468 0.056958 0.0569580.056958 propane 0.159919 0.025140 0.025140 0.025140 butane 0.0437030.003481 0.003481 0.003481 n-butane 0.082213 0.004925 0.004925 0.004925i-pentane 0.018010 0.000555 0.000555 0.000555 n-pentane 0.0072510.000174 0.000174 0.000174 n-hexane 0.000412 0.000004 0.000004 0.000004benzene 0.000054 0.000001 0.000001 0.000001 toluene 0.000000 0.0000000.000000 0.000000 p-xylene 0.000000 0.000000 0.000000 0.000000 n-heptane0.000000 0.000000 0.000000 0.000000 n-octane 0.000000 0.000000 0.0000000.000000

indicates data missing or illegible when filed

Eighth Modification of the First Embodiment

FIG. 33 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as an eighth modification of thefirst embodiment of the present invention. In the liquefaction systemillustrated in FIG. 33, the parts corresponding to those of theliquefaction system 1 of the first embodiment (including themodifications thereof) are denoted with like numerals and omitted fromthe following discussion except for the matters that will be discussedin the following.

The eighth modification is similar to the fifth modification discussedabove, but the fifth cooler 80 of the fifth modification is omitted, anda heat exchanger 100 is added between the line L6 leading from thedistillation unit 15 and the line L10 leading from the first compressor4. As a result, the material gas expelled from the first compressor 4 tothe line L10 is cooled by the material gas (top fraction) expelled fromthe distillation unit 15 to the line L6, instead of being cooled by thefifth cooler 80, and is introduced into a heat exchanger 79 similarly tothat of the fifth modification. Meanwhile, the material gas expelledfrom the distillation unit 15 is introduced into the liquefaction unit21 via the line L6 following the heat exchange, and is then cooled bythe piping system 22. Owing to this arrangement, in the eighthmodification, the cooling of the material gas by the fifth cooler 80 asin the fifth embodiment may be augmented or replaced by the heatexchange in the heat exchanger 100. In the eighth embodiment, the heatexchanger 69 that was used in the fifth embodiment is omitted, but it isalso possible to arrange such that the material gas expelled from thedistillation unit 15 to the line L6 is introduced into the heatexchanger 100 via the heat exchanger 69.

Second Embodiment

FIG. 11 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a second embodiment of thepresent invention. Table 7 shows the temperature, the pressure, the flowrate and the molar composition of the natural gas that is to beliquefied at each of various points in the liquefaction system of thesecond embodiment by way of an example. In the liquefaction systemillustrated in FIG. 11, the parts corresponding to those of theliquefaction system 1 of the first embodiment are denoted with likenumerals and omitted from the following discussion except for thematters that will be discussed in the following.

The liquefaction system 1 of the second embodiment further includes afourth compressor 71 for gas supply and a fourth cooler 72 on theupstream end of the line L1 for supplying the material gas to the waterremoval unit 2. In this liquefaction system 1, the material gas suppliedfrom a line L18 is compressed by the fourth compressor 71 for gassupply, and cooled by the fourth cooler 72 connected to the downstreamend thereof before being supplied to the water removal unit 2.

In this liquefaction system 1 of the second embodiment, even when thepressure of the material gas that is supplied to the liquefaction system1 is relatively low, the material gas can be compressed to a desiredpressure by the fourth compressor 71 for gas supply so that the materialgas that is supplied from the first compressor 4 to the liquefactionunit 21 can be maintained at a relatively high pressure level (about6,800 kPaA in this case). This liquefaction system 1 is particularlysuitable for processing material gas from a source of a relatively lowpressure such as shale gas.

Also, because the liquefaction system 1 of the second embodiment canmaintain the temperature of the material gas that is supplied from thefirst compressor 4 to the liquefaction unit 21 at a relatively highlevel, owing to the presence of the fourth compressor 71 for gas supply,the line L10 may be connected to the piping system 30 positioned in awarm part or the warm region Z1 of the liquefaction unit 21 (the pointof introducing the mixed refrigerant having a substantially sametemperature level as the material gas that is introduced into theliquefaction unit 21). Thereafter, the material gas is caused to flowfrom the piping system 30 to the piping system 31 positioned in theintermediate region Z2 and thence to the piping system 32 positioned inthe cold region Z3 to be liquefied and super cooled.

Thus, in the liquefaction system 1 of the second embodiment, even whenthe temperature of the material gas that is introduced into theliquefaction unit 21 should rise, because the material gas is introducedinto the warm region Z1 (high temperature side) of the liquefaction unit21 having a similar temperature level, the thermal load (thermalstresses) on the liquefaction unit 21 can be reduced, and the efficiencyof the liquefaction process can be increased. The liquefaction system 1can be configured such that the material gas is introduced into the warmregion Z1 of the liquefaction unit 21, without regard to the presence ofthe fourth compressor 71 for gas supply, depending on the pressure levelof the material gas. If the pressure of the material gas is so high thatthe temperature of the material gas is higher than the warm region Z1(high temperature side) of the liquefaction unit 21, the load on theliquefaction unit 21 can be reduced by providing the second cooler 85similarly as in the embodiment illustrated in FIG. 10.

TABLE 7 No. (i) (ii) (iii) (iv) vapor phase faction 1.00 0.999268140.99999155 0.00 temperature [C.] 20.14 −4.64 −42.58 175.78 pressure[kPa] 7180.00 4850.00 4700.00 4705.00 molar flow rate 42000 42000 45020312.584899 [kgmole/h] mass flow rate [kg/

719619 719619 783495 19781 molar fraction nitrogen 0.0081995900.000031631 methane 0.949952502 0.041864778 ethane 0.0249987500.032036256 propane 0.009999500 0.144339655 butane 0.0019999000.165884716 n-butane 0.001999900 0.233258825 i-pentane 0.0004999750.066912683 n-pentane 0.000499975 0.067112604 n-hexane 0.0005999700.080613490 benzene 0.000499975 0.067177784 toluene 0.0000999950.013435677 p-xylene 0.000049998 0.006717839 n-heptane 0.0004999750.067178385 n-octane 0.000099995 0.013435678 No. (v) (vi) (vii) (viii)(ix) vapor phase faction 0.926255 0.00 1.00 1.00 0.00 temperature [C.]−65.19 −65.19 −65.19 −38.31 −161.55 pressure [kPa] 4400.00 4400.004400.00 6799.08 120.00 molar flow rate 45020 3320 41700 41700 41700[kgmole/h] mass flow rate [kg/

783495 83262 700225 698733 698733 molar fraction nitrogen 0.008252582methane 0.956625333 ethane 0.024955530 propane 0.009098899 butane0.000795603 n-butane 0.000269470 i-pentane 0.000001763 n-pentane0.000000820 n-hexane 0.000000000 benzene 0.000000000 toluene 0.000000000p-xylene 0.000000000 n-heptane 0.000000000 n-octane 0.000000000

indicates data missing or illegible when filed

Third Embodiment

FIG. 12 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a third embodiment of thepresent invention. Table 8 shows the temperature, the pressure, the flowrate and the molar composition of the natural gas that is to beliquefied at each of various points in the liquefaction system of thethird embodiment by way of an example. In the liquefaction systemillustrated in FIG. 12, the parts corresponding to those of theliquefaction systems 1 of the first and second embodiments are denotedwith like numerals and omitted from the following discussion except forthe matters that will be discussed in the following.

The liquefaction system 1 of the third embodiment further includes asecond compressor 75 for additional compression connected to thedownstream end of the first compressor 4 so that the material gas whichhas been compressed by the first compressor 4 is forwarded to the secondcompressor 75 via a line L10 a, and after being further compressed (toabout 7,000 kPaA in this case) in the second compressor 75, isintroduced into the liquefaction unit 21 via a line L10 b. The internalstructure of the liquefaction unit 21 is similar to that of the secondembodiment, and the line L10 b is connected to a piping system 30positioned in the warm region Z1 of the liquefaction unit 21.

In the liquefaction system 1 of the third embodiment, because the secondcompressor 75 is added to the downstream end of the first compressor 4,the pressure of the material gas that is forwarded from the secondcompressor 75 to the liquefaction unit 21 via the line L10 b can beincreased even further (up to 7,000 to 10,000 kPaA, for instance) sothat the efficiency of the liquefaction process can be increased evenfurther.

TABLE 8 No. (i) (ii) (iii) (iv) (v) vapor phase faction 1.00 1.000.99998949 0.00 0.925944 temperature [C.] 20.08 8.32 −42.58 177.19−65.24 pressure [kPa] 5830.00 4850.00 4700.00 4705.00 4400.00 molar flowrate 42000 42000 45020 312.6686486 45020 [kgmole/h] mass flow rate [kg/

719619 719619 783504 19764 783504 molar fraction nitrogen 0.0081995900.000033626 methane 0.949952502 0.043508871 ethane 0.0249987500.032339550 propane 0.009999500 0.143654595 butane 0.0019999000.165149865 n-butane 0.001999900 0.232835468 i-pentane 0.0004999750.066891831 n-pentane 0.000499975 0.067093928 n-hexane 0.0005999700.080591893 benzene 0.000499975 0.067159786 toluene 0.0000999950.013432078 p-xylene 0.000049998 0.006716040 n-heptane 0.0004999750.067160391 n-octane 0.000099995 0.013432079 No. (vi) (vii) (viii) (ix)vapor phase faction 0.00 1.00 1.00 0.00 temperature [C.] −65.24 −65.24−34.55 −161.55 pressure [kPa] 4400.00 4400.00 7000.00 120.00 molar flowrate 3334 41686 41686 41700 [kgmole/h] mass flow rate [kg/

83548 699948 699948 698733 molar fraction nitrogen 0.008260844 methane0.956667221 ethane 0.024931118 propane 0.009076200 butane 0.000793571n-butane 0.000268518 i-pentane 0.000001710 n-pentane 0.000000817n-hexane 0.000000000 benzene 0.000000000 toluene 0.000000000 p-xylene0.000000000 n-heptane 0.000000000 n-octane 0.000000000

indicates data missing or illegible when filed

Modification of the Third Embodiment

FIG. 13 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a modification of the thirdembodiment of the present invention. In the liquefaction systemillustrated in FIG. 13, the parts corresponding to those of theliquefaction system 1 of the first to the third embodiments are denotedwith like numerals and omitted from the following discussion except forthe matters that will be discussed in the following.

In the liquefaction system of this modification, the second compressor75 is driven by an electric motor (first electric motor) 81, and thespeed of the electric motor 81 is controlled by a controller 82 designedfor variable frequency drive. The electric motor 81 receives an externalsupply of electric power. The speed of the electric motor 81 (or theoperation of the second compressor 75) is controlled according to thepressure value detected by a pressure gauge 83 provided in the line L10b so that the pressure of the material gas that is introduced into theliquefaction unit 21 is maintained at a fixed value (or within a fixedrange). As a result, the pressure of the material gas that is introducedinto the liquefaction unit 21 can be increased by the second compressor75 in a stable manner so that the temperature of the material gas isalso maintained within an appropriate range, and the liquefactionprocess in the liquefaction unit 21 can be carried out in a bothefficient and stable manner.

Fourth Embodiment

FIG. 14 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fourth embodiment of thepresent invention. Table 9 shows the temperature, the pressure, the flowrate and the molar composition of the natural gas that is to beliquefied at each of various points in the liquefaction system of thefourth embodiment by way of an example. In the liquefaction systemillustrated in FIG. 14, the parts corresponding to those of theliquefaction system 1 of the first to third embodiments are denoted withlike numerals and omitted from the following discussion except for thematters that will be discussed in the following.

The liquefaction system 1 of the fourth embodiment further includes asecond cooler 85 using a low pressure (LP) propane refrigerant (C3R)provided on the downstream end of the second compressor 75 of the thirdembodiment shown in FIG. 12. The material gas that is expelled from thefirst compressor 4 to the line L10 a is compressed by the secondcompressor 75, forwarded to the second cooler 85 to be cooled thereby,and introduced into the liquefaction unit 21 via a line L10 c. Theinternal structure of the liquefaction unit 21 is similar to that of thethird embodiment, and the line L10 c is connected to a piping system 30positioned in the warm region Z1 of the liquefaction unit 21.

In the liquefaction system 1 of the fourth embodiment, owing to thecompression of the material gas by the second compressor 75, even whenthe temperature of the material gas should exceed an appropriate range,by cooling the material gas in the second cooler 85 provided downstreamof the second compressor 75 by using a low pressure propane refrigerant,the temperature of the material gas can be brought close to thetemperature level of the warm region Z1 of the liquefaction unit 21 sothat the thermal load on the liquefaction unit 21 can be reduced, andthe efficiency of the liquefaction process can be increased. If thesecond cooler 85 (using a propane refrigerant demonstrating a highercooling capability than water or air) is used for the cooling of thematerial gas in the recycle operation at the time of the startup of thefirst compressor 4, an improved cooling (below 0 degrees Celsius)performance can be achieved.

TABLE 9 No. (i) (ii) (iii) (iv) (v) vapor phase faction 1.00 1.000.99998949 0.00 0.925944 temperature [C.] 20.08 8.32 −42.58 177.19−65.24 pressure [kPa] 5830.00 4850.00 4700.00 4705.00 4400.00 molar flowrate 42000 42000 45020 312.6686486 45020 [kgmole/h] mass flow rate [kg/

719619 719619 783504 19764 783504 molar fraction nitrogen 0.0081995900.000033626 methane 0.949952502 0.043508871 ethane 0.0249987500.032339550 propane 0.009999500 0.143654595 butane 0.0019999000.165149865 n-butane 0.001999900 0.232835468 i-pentane 0.0004999750.066891831 n-pentane 0.000499975 0.067093928 n-hexane 0.0005999700.080591893 benzene 0.000499975 0.067159786 toluene 0.0000999950.013432078 p-xylene 0.000049998 0.006716040 n-heptane 0.0004999750.067160391 n-octane 0.000099995 0.013432079 No. (vi) (vii) (viii) (ix)vapor phase faction 0.00 1.00 1.00 0.00 temperature [C.] −65.24 −65.24−34.50 −161.55 pressure [kPa] 4400.00 4400.00 8000.00 120.00 molar flowrate 3334 41686 41686 41700 [kgmole/h] mass flow rate [kg/

83548 699948 699948 698733 molar fraction nitrogen 0.008260844 methane0.956667221 ethane 0.024931118 propane 0.009076200 butane 0.000793571n-butane 0.000268518 i-pentane 0.000001710 n-pentane 0.000000817n-hexane 0.000000000 benzene 0.000000000 toluene 0.000000000 p-xylene0.000000000 n-heptane 0.000000000 n-octane 0.000000000

indicates data missing or illegible when filed

Table 10 compares the power requirements of the various compressors inthe first to fourth embodiments, and the first and second examples forcomparison. As shown in Table 10, the total power requirements andspecific powers of the first to fourth embodiments are less than thoseof the first and second examples for comparison (prior art).

TABLE 10 Example 1 for Example 2 for 1st 2nd 3rd 4th ComparisonComparison Embodiment Embodiment Embodiment Embodiment 1st Compressor[kW] 2493 3616 7267 3616 3616 2nd Compressor [kW] 4402 7099 4thCompressor [kW] 7561 Mixed Refrigerant 161680 155260 153620 150350148940 143590 Compressor [kW] Propane Compressor [kW] 76651 74827 7224768689 70756 72769 Total [kW] 238331 233057 225867 226600 224098 223458LNG [t/h] 698.8 694.7 698.8 698.8 698.8 698.8 Specific Power [kW/t] 341335 323 324 321 320

Fifth Embodiment

FIG. 15 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fifth embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 15,the parts corresponding to those of the liquefaction systems 1 of thefirst to fourth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

In the liquefaction system 1 of the fifth embodiment, as opposed to thefirst to fourth embodiments, the first expander 3 and the firstcompressor 4 are not mechanically connected to each other, but areelectrically connected to each other. The first expander 3 is connectedto an electric generator 87 so that the power generated by the expander3 is converted into electric power by the electric generator 87. Theelectric power generated by the electric generator 87 is supplied to anelectric motor 84 for driving the first compressor 4. In other words,the power generated by the first expander 3 is used by the firstcompressor 4. The electric power supplied by the electric generator 87may be at least a part of the electric power that is used for drivingthe electric motor 84, and when there is a shortage of electric power,the external power source may be used for augmenting the shortfall ofthe electric power.

In the liquefaction system 1 of the fifth embodiment, because the firstexpander 3 and the first compressor 4 are electrically connected to eachother, the freedom in the mode of operation of the first expander 3 andthe first compressor 4 at the time of startup and/or power-down can beincreased (such that the first expander 3 and the first compressor 4 canbe individually operated, for instance).

Sixth Embodiment

FIG. 16 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a sixth embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 16,the parts corresponding to those of the liquefaction system 1 of thefirst to fifth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

In the liquefaction system 1 of the sixth embodiment, rich gascontaining 88 mol % of methane is used as the material gas (similarly tothe modification of the sixth embodiment, and the seventh and eighthembodiments). In this liquefaction system, the material gas that isseparated as a top fraction in the distillation unit 15 is directlyintroduced into the first compressor 4 to be compressed thereby via aline L19. The material gas is then pre-cooled in the piping system 22 inthe warm region Z1, and forwarded to a first gas-liquid separationvessel 23 via a line L21.

The first gas-liquid separation vessel 23 separates a liquid phasecomponent (condensate) of the material gas, and the hydrocarbons inliquid form forming the liquid phase component is recirculated to thedistillation unit 15 via an expansion valve 89 provided in a line L22.Meanwhile, the material gas mainly consisting of methane and forming theliquid phase component in the first gas-liquid separation vessel 23 isforwarded to the piping system 31 in the liquefaction unit 21 via a lineL23.

In the liquefaction system 1 of the sixth embodiment, because the firstgas-liquid separation vessel 23 is provided on the downstream side ofthe first compressor 4, and the material gas expelled from the firstcompressor 4 is introduced into the first gas-liquid separation vessel23 via the piping system 22 positioned in the warm region Z1, thetemperature of the material gas can be brought close to the temperaturelevel of the warm region Z1 of the liquefaction unit 21. Furthermore,because the material gas is cooled in the warm region Z1 (piping system22) of the liquefaction unit 21, and the gas phase component expelledfrom the first gas-liquid separation vessel 23 is introduced into theintermediate region Z2 (piping system 31), the temperature of thematerial gas can be brought close to the temperature level of theintermediate region Z2 of the liquefaction unit 21 with ease. Also,because the material gas expelled from the first gas-liquid separationvessel 23 can be placed under pressure by the first compressor 4, therecirculation pump 24 provided in the recirculation line (line L21)extending from the first gas-liquid separation vessel 23 to thedistillation unit 15 in some of the embodiments including the firstembodiment can be omitted.

In the liquefaction of the material gas in the liquefaction unit 21,raising the outlet pressure of the compressor 4 (or increasing thepressure of the material gas that is introduced into the liquefactionunit 21) is advantageous. However, when the top fraction of thedistillation unit 15 is cooled in the liquefaction unit 21, separated inthe first gas-liquid separation vessel 23, and the separated gas phasecomponent is compressed by the first compressor 4 before beingintroduced into the liquefaction unit 21 as was the case with the firstembodiment, because the temperature of the material gas is increased bythe first compressor 4 preceding the liquefaction unit 21, depending theconditions associated with the composition, pressure and feed rate ofthe material gas, the temperature level of the material gas may deviatefrom a suitable range for introduction into the liquefaction unit 21 sothat the thermal load on the liquefaction unit 21 may become excessive.Such a problem can be resolved by changing the point of introducing thematerial gas into the liquefaction unit 21, but when the main heatexchanger consists of a kind such as a spool-wound heat exchanger whichdoes not allow the point of introduction to be changed with ease, it maynot be the case. Thus, if the material gas separated as the top fractionin the distillation unit 15 is forwarded directly to the firstcompressor 4 via the line L19 to be compressed, the material gascompressed by the first compressor 4 is cooled in the warm region Z1 ofthe liquefaction unit 21, the cooled material gas is separated in thefirst gas-liquid separation vessel 23, and the separated gas phasecomponent of the material gas is introduced into the intermediate regionZ2 (downstream of the warm region Z1) of the liquefaction unit 21, as isthe case with the present embodiment, the temperature of the materialgas can be maintained within an appropriate range (or the temperature ofthe material gas can be brought close to the temperature level at theintroduction point of the liquefaction unit 21).

First Modification of the Sixth Embodiment

FIG. 17 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a modification of the sixthembodiment of the present invention. Table 11 shows the temperature, thepressure, the flow rate and the molar composition of the natural gasthat is to be liquefied at each of various points in the liquefactionsystem of the sixth embodiment by way of an example. In the liquefactionsystem illustrated in FIG. 17, the parts corresponding to those of theliquefaction system 1 of the sixth embodiment are denoted with likenumerals and omitted from the following discussion except for thematters that will be discussed in the following.

In this liquefaction system 1 of this modification, the first cooler 11used in the sixth embodiment shown in FIG. 16 is omitted, and a secondcooler 85 using low pressure propane as the refrigerant is provided onthe downstream side of the first compressor 4. The material gas isforwarded from the first compressor 4 to the second cooler 85 to becooled therein via a line L20 a, and is forwarded to a piping system 22positioned in the warm region Z1 of the liquefaction unit 21 via a lineL20 b to be further cooled before being introduced into the firstgas-liquid separation vessel 23 via a line L21.

In this liquefaction system of the first modification of the sixthembodiment, because the second cooler 85 is provided on the downstreamside of the first compressor 4, even when the temperature of thematerial gas expelled from the first compressor 4 is higher than thetemperature in the warm region Z1 of the liquefaction unit 21, owing tothe cooling action of the second cooler 85 applied to the material gas,the temperature of the material gas can be brought close to thetemperature level of the warm region Z1 of the liquefaction unit 21.

TABLE 11 No. (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) vapor 1.000.97 1.00 0.00 1.00 0.82 0.00 1.00 0.00 phase faction temper- 20.00−15.21 −52.85 95.46 −18.62 −63.17 −63.17 −63.17 −159.04 ature [C.]pressure 7000.00 3470.10 3300.00 3310.00 5335.67 4985.67 4985.67 4985.67120.00 [kPa] molar 42000 42000 48048.7985 2599.94689 48048.8 48048.88648.78 39400 39400.01475 flow rate [kgmole/ h] mass flow 803679 803679849289 132033 849289 849289 177669 671605 671605 rate [kg/

molar fraction nitrogen 0.001000000 0.000000012 0.001066081 methane0.877900000 0.002690816 0.935674952 ethane 0.060900000 0.1364502520.055944425 propane 0.033600000 0.431375809 0.007299533 butane0.006500000 0.104814783 0.000012789 n-butane 0.011500000 0.1857424380.000002218 i-pentane 0.003400000 0.054924175 0.000000002 n-pentane0.002100000 0.033923768 0.000000000 n-hexane 0.003100000 0.0500779460.000000000 benzene 0.000000000 0.000000000 0.000000000 toluene0.000000000 0.000000000 0.000000000 p-xylene 0.000000000 0.0000000000.000000000 n-heptane 0.000000000 0.000000000 0.000000000 n-octane0.000000000 0.000000000 0.000000000

indicates data missing or illegible when filed

Second and Third Modifications of the Sixth Embodiment

FIGS. 18 and 19 are diagrams showing liquefaction process flows insystems for the liquefaction of natural gas given as a second and athird modification of the sixth embodiment of the present invention,respectively. In the liquefaction systems illustrated in FIGS. 18 and19, the parts corresponding to those of the liquefaction system 1 of thesixth embodiment (including other embodiments and modifications) aredenoted with like numerals and omitted from the following discussionexcept for the matters that will be discussed in the following.

As shown in FIG. 18, the liquefaction system 1 of the secondmodification includes a heat exchanger 69 positioned between the line L4and the line L19 so that the material gas that is separated as a topfraction in the distillation unit 15 and conducted through the line L19is heated by exchanging heat with the material gas that flows throughthe line L4 from the cooler 12 to the distillation unit 15, and is thenintroduced into the first compressor 4. The material gas which has beencompressed by the first compressor 4 is introduced into the liquefactionunit 21 via the line L20. The downstream end of the line L20 isconnected to the piping system 22 positioned in the warm region Z1 orthe warmest part of the liquefaction unit 21.

Owing to this arrangement, in the second modification, even when thetemperature of the material gas introduced into the liquefaction unit 21via the line L20 should fall below an appropriate temperature range, thetemperature of the material gas can be raised to an appropriate level bythe exchange of heat in the heat exchanger 69. In other words, thetemperature of the material gas in the line L20 which has beencompressed can be brought close to the temperature at the introductionpoint (piping system 22) in the liquefaction unit 21 so that the thermalload (thermal stresses) on the liquefaction unit 21 can be reduced.

As shown in FIG. 19, the liquefaction system 1 of the third modificationincludes a heat exchanger 69 positioned between the line L4 and the lineL20 so that the material gas expelled from the first compressor 4 andconducted through the line L20 is heated by the exchange of heat withthe material gas flowing through the line L4, and is then introducedinto the piping system positioned in the warm region Z1 of theliquefaction unit 21. In the third modification, the material gas heatedin the heat exchanger 69 is directly introduced into the liquefactionunit 21 without the intervention of the first compressor 4 so that thetemperature of the material gas that is introduced into the liquefactionunit 21 can be controlled with ease.

The positioning of the heat exchanger 69 in the second and thirdmodifications of the sixth embodiment can be changed variously withoutdeparting from the spirit of the present invention as long as thetemperature of the material gas in the line L20 following thecompression can be brought close to the temperature at the introductionpoint of the liquefaction unit 21.

FIG. 20 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a fourth modification of thesixth embodiment of the present invention. Table 12 shows thetemperature, the pressure, the flow rate and the molar composition ofthe natural gas that is to be liquefied at each of various points in theliquefaction system of the fourth modification by way of an example. Inthe liquefaction system illustrated in FIG. 20, the parts correspondingto those of the liquefaction system 1 of the sixth embodiment (includingthe other modifications) are denoted with like numerals and omitted fromthe following discussion except for the matters that will be discussedin the following.

The fourth modification is configured to be suitable when the materialgas has a relatively low pressure, and the critical pressure thereof isrelatively high owing to the composition of the material gas which mayinclude nitrogen and heavier contents, as compared with the sixthembodiment. In the liquefaction system 1, similarly to the firstmodification of the sixth embodiment, the material gas is forwarded fromthe first compressor 4 to the second cooler 85 to be cooled therein viathe line L20 a, and is introduced into the first gas-liquid separationvessel 23 via the line L20 b. However, in the fourth modification, theline L20 b is directly connected to the first gas-liquid separationvessel 23 without the intervention of the liquefaction unit 21, and thematerial gas which forms the gas phase component in the first gas-liquidseparation vessel 23 is forwarded to the piping system 30 positioned inthe warm region Z1 or the warmest part of the liquefaction unit 21.Owing to this structure, in the fourth modification, the material gasthat is introduced into the first gas-liquid separation vessel 23 is notrequired to be cooled (by introducing into the piping system 22), asopposed to the first modification so that the load on the liquefactionprocess of the liquefaction unit 21 can be reduced.

TABLE 12 No. (i) (ii) (iii) (iv) (v) vapor phase faction 1.00 0.99 1.000.00 1.00 temperature [C.] 20.0 4.2 −22.3 80.0 −6.4 pressure [kPa] 75005669 5500 5510 6814 molar flow rate 42000 42000 41420 1728 41420[kgmole/h] mass flow rate [kg/

807998 807998 757232 83213 757232 molar fraction nitrogen 0.0070000.007000 0.007152 0.000007 0.007152 methane 0.871400 0.871400 0.8932130.151770 0.893213 ethane 0.060900 0.060900 0.059298 0.149029 0.059298propane 0.033600 0.033600 0.028208 0.241401 0.028208 butane 0.0065000.006500 0.004279 0.081324 0.004279 n-butane 0.011500 0.011500 0.0064580.172432 0.006458 i-pentane 0.003400 0.003400 0.000989 0.069908 0.000989n-pentane 0.002100 0.002100 0.000385 0.046710 0.000385 n-hexane 0.0031000.003100 0.000015 0.075276 0.000015 benzene 0.000500 0.000500 0.0000020.012142 0.000002 toluene 0.000000 0.000000 0.000000 0.000000 0.000000p-xylene 0.000000 0.000000 0.000000 0.000000 0.000000 n-heptane 0.0000000.000000 0.000000 0.000000 0.000000 n-octane 0.000000 0.000000 0.0000000.000000 0.000000 No. (vi) (vii) (viii) (ix) vapor phase faction 0.970.00 1.00 0.00 temperature [C.] −34.5 −34.5 −34.5 −160.9 pressure [kPa]6749 6749 6749 120 molar flow rate 41420 1146 40274 40274 [kgmole/h]mass flow rate [kg/

757232 32372 724861 724861 molar fraction nitrogen 0.007152 0.0619790.007300 0.007300 methane 0.893213 0.575496 0.902252 0.902252 ethane0.059298 0.135734 0.057123 0.057123 propane 0.028208 0.151884 0.0246890.024689 butane 0.004279 0.039011 0.003291 0.003291 n-butane 0.0064580.071862 0.004598 0.004598 i-pentane 0.000989 0.016367 0.000552 0.000552n-pentane 0.000385 0.007216 0.000191 0.000191 n-hexane 0.000015 0.0003960.000005 0.000005 benzene 0.000002 0.000056 0.000001 0.000001 toluene0.000000 0.000000 0.000000 0.000000 p-xylene 0.000000 0.000000 0.0000000.000000 n-heptane 0.000000 0.000000 0.000000 0.000000 n-octane 0.0000000.000000 0.000000 0.000000

indicates data missing or illegible when filed

Seventh Embodiment

FIG. 21 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a seventh embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 21,the parts corresponding to those of the liquefaction system 1 of thefirst to sixth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

The liquefaction system 1 of the seventh embodiment is similar to thatof the sixth embodiment, but differs therefrom in that two expanders(first expander 3 a and second expander 3 b) are connected to thedownstream end of the water removal unit 2 in parallel to each other. Inthe seventh embodiment, the first expander 3 a and the second expander 3b are connected to a pair of compressors (first compressor 4 a and thirdcompressor 4 b), respective, via a common shafts 5 a, 5 b in each case.

As shown in FIG. 21, the material gas expelled from the water removalunit 2 is forwarded to the first and second expanders 3 a and 3 b viarespective lines L2 a and L2 b. The material gas expelled from the firstand second expanders 3 a and 3 b is forwarded to the cooler 12 via linesL3 a, L3 b and L3. In this case, because the required cooling capacityof the cooling unit can be reduced, only a single cooler 12 using a lowpressure (LP) propane refrigerant (C3R) is provided.

The material gas separated as a top fraction of the distillation unit 15is forwarded to the third compressor 4 b via a line L19 to becompressed. The material gas is then forwarded from the third compressor4 b to a piping system 22 positioned in the warm region Z1 to be cooledtherein via the line L20, and is then introduced into the firstgas-liquid separation vessel 23 via a line L21.

The first gas-liquid separation vessel 23 separates the liquid phasecomponent (condensate) of the material gas, and the liquid phasecomponent which is formed by hydrocarbons in liquid form is recirculatedto the distillation unit 15 via an expansion valve 89 provided in a lineL22. Meanwhile, the material gas that forms the gas phase componentseparated in the first gas-liquid separation vessel 23 is forwarded tothe first compressor 4 a via a line L24 to be compressed, and thematerial gas expelled from the first compressor 4 a is introduced into apiping system 30 positioned in the warm region Z1 of the liquefactionunit 21 via a line L25.

According to the arrangement of the seventh embodiment using a pair ofexpanders 3 a and 3 b and a pair of compressors 4 a and 4 b, even whenthe material gas supplied to the liquefaction system 1 has a relativelyhigh pressure and has a low critical pressure, the material gas can becompressed in an appropriate manner (without causing the material gasthat is introduced into the distillation unit 15 to be compressed beyondthe critical pressure) by using a plurality of compressors 4 a and 4 b.

Eighth Embodiment

FIG. 22 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as an eighth embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 22,the parts corresponding to those of the liquefaction system 1 of thefirst to seventh embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

The liquefaction system 1 of the eighth embodiment is similar to that ofthe sixth or the seventh embodiment, but differs therefrom in that thetwo first expanders 3 a and 3 b are connected in series, and a separator91 is positioned between the two first expanders 3 a and 3 b.

As shown in FIG. 22, the material gas expelled from the water removalunit 2 is forwarded to the first expander 3 a via a line L2 to beexpanded therein, and is introduced into the separator 91 via a line L3.The material gas that is separated as a gas phase component in theseparator 91 is forwarded to the second expander 3 b via a line L26 tobe expanded therein, and is forwarded to a cooler 12 via a line L27.Meanwhile, the liquid phase component (condensate) of the material gasis forwarded to the cooler 12 via an expansion valve 92 provided in aline L28.

According to the eighth embodiment, similarly to the seventh embodimentdiscussed above, even when the material gas supplied to the liquefactionsystem has a relatively high pressure and has a low critical pressure,the material gas can be compressed in an appropriate manner by using aplurality of compressors 4 a and 4 b.

First and Second Modifications of the Eighth Embodiment

FIGS. 23 and 24 are diagrams showing liquefaction process flows insystems for the liquefaction of natural gas given as a first and asecond modification of the eighth embodiment of the present invention,respectively. In the liquefaction systems illustrated in FIGS. 23 and24, the parts corresponding to those of the liquefaction system 1 of theeighth embodiment (including other embodiments and modifications) aredenoted with like numerals and omitted from the following discussionexcept for the matters that will be discussed in the following.

As shown in FIG. 23, the liquefaction system 1 of the first modificationis similar to the eighth embodiment, but differs therefrom in that theheat exchanger 69 is provided between the line L4 and the line L19.Therefore, the material gas separated as a top fraction in thedistillation unit 15 and conducted through the line L19 is heated byexchanging heat with the material gas that flows through the line L4leading from the cooler 12 to the distillation unit 15 before beingintroduced into the third compressor 4 b. In the first modification,owing to this arrangement, even when the temperature of the material gasthat is introduced into the liquefaction unit 21 via the line L20 shouldfall below an appropriate range, the temperature of the material gas canbe raised appropriately by the exchange of heat in the heat exchanger69.

As shown in FIG. 24, the liquefaction system 1 of the secondmodification is similar to the eighth embodiment, but differs therefromin that the heat exchanger 69 is provided between the line L4 and theline L25. Therefore, the material gas compressed by the first compressor4 a and conducted through the line L25 is heated by exchanging heat withthe material gas that flows through the line L4 leading from the cooler12 to the distillation unit 15 before being introduced into the pipingsystem 30 positioned in the warm region Z1 of the liquefaction unit 21.In the second modification, because the material gas that is heated inthe heat exchanger 69 is directly introduced into the liquefaction unit21 without the intervention of the first compressor 4, the temperatureof the material gas that is introduced into the liquefaction unit can becontrolled with ease.

The positioning of the heat exchanger 69 in the first and secondmodifications can be changed variously without departing from the spiritof the present invention as long as the temperature of the material gasthat is to be introduced into the liquefaction unit 21 can be broughtclose to the temperature at the introduction point of the liquefactionunit 21.

Ninth Embodiment

FIG. 25 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a ninth embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 25,the parts corresponding to those of the liquefaction system 1 of thefirst to eighth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

The liquefaction system 1 of the ninth embodiment is advantageous inarrangements similar to the first modification of the sixth embodimentwhen the critical pressure of the material gas is relatively low and thepressure of the material gas expelled from the first compressor 4 to thefirst gas-liquid separation vessel 23 may be higher than the criticalpressure (or when the first gas-liquid separation vessel 23 is unable tofunction properly). In this liquefaction system 1, the material gas isforwarded from a first compressor 4 to a second cooler 85 via a line L20a to be cooled therein, and is then forwarded to a piping system 22positioned in the warm region Z1 of the liquefaction unit 21 via a lineL20 b to be further cooled therein. The material gas conducted throughthe line L21 is forwarded to lines L22 and L23 which branch out from abranch point of the line L21 one above the other so that a part of thematerial gas is recirculated to the distillation unit 15 via anexpansion valve 89 provided in the lower line L22, and the remainingpart of the material gas is introduced into the piping system 31positioned in the intermediate region Z2 of the liquefaction unit 21 viathe upper line L23. Owing to this arrangement, the liquefaction system 1of the ninth embodiment allows the load on the liquefaction process inthe liquefaction unit 21 to be reduced.

Modification of the Ninth Embodiment

FIG. 26 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a modification of the ninthembodiment of the present invention. In the liquefaction systemillustrated in FIG. 26, the parts corresponding to those of theliquefaction system 1 of the ninth embodiment are denoted with likenumerals and omitted from the following discussion except for thematters that will be discussed in the following.

The liquefaction system 1 of this modification includes a secondgas-liquid separation vessel 25 into which the material gas conductedthrough the line L22 is introduced via an expansion valve 89. The secondgas-liquid separation vessel 25 separates the liquid phase component ofthe material gas, and recirculates the separated liquid phase componentto the distillation unit 15 via an expansion valve 90 provided in a lineL30. Meanwhile, the material gas that forms the gas phase component inthe second gas-liquid separation vessel 25 is forwarded to a line L31which is connected to a line L19 so that the material gas is forwardedto the first compressor 4 via an expansion valve 93 provided in the lineL31. Owing to this arrangement, the liquefaction system 1 of thismodification has the advantage of stabilizing the process in thedistillation unit 15.

Tenth Embodiment

FIG. 27 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as a tenth embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 27,the parts corresponding to those of the liquefaction system 1 of thefirst to ninth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

The liquefaction system 1 of the tenth embodiment is similar to thesixth embodiment shown in FIG. 16, but is similar to the example forcomparison shown in FIG. 3 as far as the upstream part of thedistillation unit 15 is concerned. More specifically, in theliquefaction system 1 of the tenth embodiment, the expander 3 ispositioned on the downstream side of the cooling unit (three coolers 10,11 and 12 in this case), and the material gas expelled from the cooler12 is forwarded to the separator 13 via the line L4 a to be separatedinto gas and liquid. The material gas that forms the gas phase componentin the separator 13 is forwarded to the expander 3 via the line L4 b,and after being expanded in the expander 3, is forwarded to thedistillation unit 15 via the line L4 c. The material gas that forms theliquid phase component in the separator 13 is forwarded to the line L4 dprovide with an expansion valve 14. After being expanded in theexpansion valve 14, the liquid phase component is forwarded to thedistillation unit 15 via the line L4 c along with the material gas fromthe expander 3.

In the liquefaction system 1 of the tenth embodiment, owing to thisarrangement, by positioning the expander 3 on the downstream side of thecooling unit so as to reduce the output power thereof, the excessiverise in the temperature of the material gas that is compressed by thecompressor 4 using the power provided by the expander 3 can be avoidedso that the temperature of the material gas can be easily brought closeto the temperature at the introduction point of the liquefaction unit 21with ease. The advantage gained by the sixth embodiment can also begained without regard to the arrangement of the first expander 3 and thecoolers 11 and 12 (the cooler 10 is omitted in the sixth embodiment). Inthe liquefaction system 1 of the tenth embodiment, similarly as in theembodiment discussed in conjunction with the embodiment illustrated inFIG. 17, a second cooler 85 using lower pressure propane as therefrigerant may be optionally provided on the downstream end of thefirst compressor 4. Similarly as in the embodiment illustrated in FIG.26, instead of the first gas-liquid separation vessel 23, a secondgas-liquid separation vessel 25 may be provided in this liquefactionsystem 1 for receiving the material gas conducted through the line L22via an expansion valve 89. In such a case, the structure surrounding thesecond gas-liquid separation vessel 25 (such as the lines L30 and L31,and the expansion valves 89 and 90) may be similar to that shown in FIG.26.

First and Second Modifications of the Tenth Embodiment

FIGS. 28 and 29 are diagrams showing liquefaction process flows insystems for the liquefaction of natural gas given as a first and asecond modification of the tenth embodiment of the present invention,respectively. In the liquefaction systems illustrated in FIGS. 28 and29, the parts corresponding to those of the liquefaction system 1 of thetenth embodiment are denoted with like numerals and omitted from thefollowing discussion except for the matters that will be discussed inthe following.

As shown in FIG. 28, the liquefaction system 1 of the first modificationis similar to the tenth embodiment, but differs therefrom in that a heatexchanger 69 is provided between the line L4 a and the line L19 so thatthe material gas that is separated as a top fraction in the distillationunit 15 and conducted through the line L19 is heated by exchanging heatwith the material gas that flows through the line L4 a from the cooler12 to the separator 13, on the upstream side of the first expander 3,and is then introduced into the first compressor 4. Owing to thisarrangement, in the first modification, even when the temperature of thematerial gas that is introduced into the liquefaction unit 21 via theline L20 should fall below an appropriate range, the temperature of thematerial gas can be maintained at an appropriate level by the exchangeof heat in the heat exchanger 69.

As shown in FIG. 29, the liquefaction system 1 of the secondmodification is similar to the tenth embodiment, but differs therefromin that the heat exchanger 69 is provided between the line L4 a and theline L25. Therefore, the material gas compressed by the first compressor4 a and conducted through the line L20 is heated by exchanging heat withthe material gas that flows through the line L4 a leading from thecooler 12 to the separator 13 before being introduced into the pipingsystem 22 positioned in the warm region Z1 of the liquefaction unit 21.In the second modification, because the material gas that is heated inthe heat exchanger 69 is directly introduced into the liquefaction unit21 without the intervention of the first compressor 4, the temperatureof the material gas that is introduced into the liquefaction unit can becontrolled with ease.

Eleventh Embodiment

FIG. 30 is a diagram showing a liquefaction process flow in a system forthe liquefaction of natural gas given as an eleventh embodiment of thepresent invention. In the liquefaction system illustrated in FIG. 30,the parts corresponding to those of the liquefaction system 1 of thefirst to tenth embodiments are denoted with like numerals and omittedfrom the following discussion except for the matters that will bediscussed in the following.

The liquefaction system 1 of the eleventh embodiment is similar to thesixth embodiment discussed above, but differs therefrom in that thefirst expander 3 is connected to the first compressor 4 similarly as inthe fifth embodiment illustrated in FIG. 15. More specifically, in theliquefaction system 1 of the eleventh embodiment, the first expander 3and the first compressor 4 are not mechanically connected to each other,but are electrically connected to each other. The first expander 3 isconnected to an electric generator 87, and the power generated by thefirst expander 3 is converted into electric power by this electricgenerator 87. The electric power generated by the electric generator 87is supplied to an electric motor 84 that drives the first compressor 4.In other words, the power generated by the first expander 3 is used bythe first compressor 4. The electric power supplied by the electricgenerator 87 may be at least a part of the electric power that is usedfor driving the electric motor 84, and when there is a shortage ofelectric power, the external power source may be used for augmenting theshortfall of the electric power.

(Modifications of the Expander and the Compressor)

FIGS. 31 and 32 are diagram showing a first and a second variation ofthe mechanical connecting arrangement between the expander and thecompressor in the system for the liquefaction of natural gas that may beuse in the various embodiments discussed above.

In the variation illustrated in FIG. 31, an electric motor (secondelectric motor) 84 is interposed between the first expander 3 and thefirst compressor 4, and the speed of the electric motor 84 is controlledby a controller 82 for variable frequency control drive. The electricmotor 84 receives a supply of electric power from an external source.The first expander 3, the first compressor 4 and the electric motor 84are provided on a common shaft, and the power generated by the firstexpander 3 by the expansion of the material gas can be used for drivingthe first compressor 4. Thereby, the power requirement of the electricmotor 84 can be reduced. By thus using the power of the electric motor84 for augmenting the power generated by the first expander 3, theoutlet pressure of the first compressor 4 can be increased in a stablemanner.

In the variation illustrated in FIG. 32, the shafts of the firstexpander 3, the first compressor 4 and the electric motor 84 are fittedwith gears 96, 97 and 98, respectively. The gear 96 of the firstexpander 3 meshes with the gear 97 of the electric motor 84, and thegear 97 of the electric motor 84 meshes with the gear 98 of the firstcompressor 4. Thus, the first expander 3 and the first compressor 4 areconnected in a power transmitting relationship (connected mechanically)via the electric motor 84. Owing to this arrangement, by using the powerof the electric motor 84 to augment the power generated by the firstexpander 3, the outlet pressure of the first compressor 4 can beincreased in a stable manner. The connecting arrangement between thefirst expander 3, the first compressor 4 and the electric motor 84 mayconsist of any per se known gear mechanisms such as a planetary gearmechanism.

The present invention has been described in terms of specificembodiments, but these embodiments are only examples, and do not limitthe present invention in any way. The various components of theliquefaction systems and the liquefaction methods for the liquefactionof the natural gas according to the present invention are notnecessarily entirely indispensable, but may be suitably substituted andomitted without departing from the spirit of the present invention.

GLOSSARY

-   1 liquefaction system-   2 water removal unit-   3, 3 a first expander-   3 b second expander-   4, 4 a first compressor-   4 b third compressor-   5 shaft-   10, 11, 12 first cooler-   15 distillation unit-   21 liquefaction unit-   23 first gas-liquid separation vessel-   33 expansion valve-   41 refrigerant separator-   44 expansion valve-   45 spray header-   54 expansion valve-   55 spray header-   69 heat exchanger-   71 fourth compressor-   72 fourth cooler-   75 second compressor-   81 electric motor (first electric motor)-   82 controller-   83 pressure gauge-   84 electric motor (second electric motor)-   85 second cooler-   86 third cooler-   87 electric generator-   89 expansion valve-   91 separator-   92 expansion valve-   96, 97, 97 gear-   Z1 warm region-   Z2 intermediate region-   Z3 cold region

1. A system for the liquefaction of natural gas that cools the naturalgas to produce liquefied natural gas, comprising: a first expander forgenerating power by expanding natural gas under pressure as materialgas; a first cooling unit for cooling the material gas depressurized byexpansion in the first expander; a distillation unit for reducing oreliminating a heavy component in the material gas by distilling thematerial gas cooled by the first cooling unit; a first compressor forcompressing the material gas from which the heavy component was reducedor eliminated by the distillation unit by using the power generated inthe first expander; and a liquefaction unit for liquefying the materialgas compressed by the first compressor by exchanging heat with arefrigerant.
 2. The system for the liquefaction of natural gas accordingto claim 1, further comprising a second cooling unit placed between thefirst compressor and the liquefaction unit to cool the material gascompressed by the first compressor.
 3. The system for the liquefactionof natural gas according to claim 1, wherein the liquefaction unitcomprises a spool-wound heat exchanger, and the material gas expelledfrom the first compressor is introduced into a warm region of thespool-wound heat exchanger located on a hot side of the spool-wound heatexchanger.
 4. The system for the liquefaction of natural gas accordingto claim 1, further comprising a second compressor placed between thefirst compressor and the liquefaction unit for compressing the materialgas expelled from the first compressor.
 5. The system for theliquefaction of natural gas according to claim 4, further comprising afirst electric motor powered by an external electric power andcontrolled in dependence on a pressure value of the material gasintroduced into the liquefaction unit, and the second compressor isdriven by the first motor.
 6. The system for the liquefaction of naturalgas according to claim 4, further comprising a second cooling unitplaced between the second compressor and the liquefaction unit to coolthe material gas.
 7. The system for the liquefaction of natural gasaccording to claim 1, further comprising an electric generator unit forconverting the power generated by the first expander into electric powerand a second electric motor for driving the first compressor, the secondelectric motor being powered by electric power generated by thegenerator unit.
 8. The system for the liquefaction of natural gasaccording to claim 1, further comprising a second electric motormechanically coupling the first expander and the first compressor toeach other and powered by external electric power, wherein the firstcompressor is configured to compress the material gas by using the powergenerated by the first expander and power generated by the secondelectric motor.
 9. The system for the liquefaction of natural gasaccording to claim 1, wherein the material gas from which the heavycomponent is reduced or eliminated by the distillation unit is directlyintroduced into the first compressor, and the system further comprises afirst gas-liquid separation vessel for receiving the material gascompressed by the first compressor via the liquefaction unit; andwherein a gas phase component of the material gas separated in the firstgas-liquid separation vessel is introduced into the liquefaction unitonce again, and a liquid phase component of the material gas isrecirculated to the distillation unit.
 10. The system for theliquefaction of natural gas according to claim 9, further comprising asecond cooling unit placed between the first compressor and the firstgas-liquid separation vessel to cool the material gas.
 11. The systemfor the liquefaction of natural gas according to claim 1, furthercomprising: a second expander placed between the first expander and thedistillation unit to generate power by expanding the material gas; and athird compressor placed between the distillation unit and the firstcompressor to compress the material gas distilled by the distillationunit by using the power generated by the second expander.
 12. The systemfor the liquefaction of natural gas according to claim 1, furthercomprising: a second expander placed in parallel with the first expanderto generate power by expanding the material gas; and a third compressorplaced between the distillation unit and the first compressor tocompress the material gas distilled by the distillation unit by usingthe power generated by the second expander.
 13. The system for theliquefaction of natural gas according to claim 1, wherein theliquefaction unit comprises a plate-fin heat exchanger.
 14. The systemfor the liquefaction of natural gas according to claim 1, wherein thematerial gas compressed by the first expander has a pressure higher than5,171 kPaA.
 15. The system for the liquefaction of natural gas accordingto claim 4, wherein the material gas compressed by the second expanderhas a pressure higher than 5,171 kPaA.
 16. The system for theliquefaction of natural gas according to claim 1, further comprising aheat exchanger for exchanging heat between the material gas introducedinto the distillation unit and a top fraction from the distillationunit.
 17. The system for the liquefaction of natural gas according toclaim 1, further comprising a first gas-liquid separation vessel forreceiving a top fraction from the distillation unit, and a third coolingunit placed between the distillation unit and the first gas-liquidseparation vessel to cool the top fraction from the distillation unit.18. The system for the liquefaction of natural gas according to claim 1,further comprising a second heat exchanger for exchanging heat betweenthe material gas to be introduced into the first compressor and thematerial gas compressed by the first compressor.
 19. The system for theliquefaction of natural gas according to claim 18, further comprising afifth cooling unit for cooling the material gas compressed by the firstcompressor at a point upstream of the second heat exchanger by using awater, air or propane refrigerant.
 20. The system for the liquefactionof natural gas according to claim 18, further comprising a third heatexchanger for exchanging heat between the material gas compressed by thefirst compressor and the top fraction from the distillation unit.
 21. Asystem for the liquefaction of natural gas that cools the natural gas toproduce liquefied natural gas, comprising: a first expander forgenerating power by expanding natural gas under pressure as materialgas; a distillation unit for reducing or eliminating a heavy componentin the material gas by distilling the material gas depressurized byexpansion in the first expander; a first compressor for compressing thematerial gas from which the heavy component was reduced or eliminated bythe distillation unit by using power generated in the first expander;and a liquefaction unit for liquefying the material gas compressed bythe first compressor by exchanging heat with a refrigerant.
 22. A methodfor the liquefaction of natural gas by cooling the natural gas toproduce liquefied natural gas, comprising: a first expansion step forgenerating power by expanding natural gas under pressure as materialgas; a first cooling step for cooling the material gas depressurized byexpansion in the first expansion step; a distillation step for reducingor eliminating a heavy component in the material gas by distilling thematerial gas cooled in the first cooling step; a first compression stepfor compressing the material gas from which the heavy component wasreduced or eliminated in the distillation step by using the powergenerated in the first expansion step; and a liquefaction step forliquefying the material gas compressed in the first compression step byexchanging heat with a refrigerant.